The Sumitomo Copper Scandal

Yasuo Hamanaka was Sumitomo Corporation’s master copper trader—so dominant that London brokers called him “Mr. Copper” and acknowledged that he was the global copper market. For over a decade, Hamanaka deployed Sumitomo’s balance sheet to accumulate and lock up copper supplies, derivatives positions, and trading secrets, betting that his sheer firepower could defy the laws of supply and demand. In June 1996, the scheme unravelled when Sumitomo revealed a $2.6 billion loss and Hamanaka’s hidden positions. The episode exposed how a single trader, armed with corporate capital and minimal oversight, could manipulate global commodity prices for years—and how that manipulation had a brutal price.

How one trader became the copper market

In the early 1980s, Yasuo Hamanaka was a mid-level trader at Sumitomo Corporation’s copper desk in Tokyo. The global copper market was fragmented: physical traders, smelters, fabricators, and financial speculators all competed on exchanges in London and New York. Hamanaka’s insight was that copper, unlike wheat or oil, was concentrated in the hands of a few mining companies and industrial consumers. Sumitomo, with deep industrial connections across Japan, had the balance sheet to become a dominant buyer and holder. By controlling physical inventory and massive derivatives positions simultaneously, Hamanaka theorized he could nudge prices in a favourable direction and earn outsized trading profits.

He began accumulating. Sumitomo bought physical copper—bars, cathodes, whatever was available. Hamanaka also took enormous long positions in futures contracts and forward contracts on the London Metal Exchange (LME), betting that copper would rise. The strategy required capital, secrecy, and an absolute conviction that prices would break in his direction. Sumitomo’s treasury supplied the first; Hamanaka’s opaque position-keeping supplied the second.

By the late 1980s, Hamanaka had accumulated nearly 5% of the world’s entire copper supply. Brokers deferred to him on pricing. Other traders hedged their own positions by watching his every move. The market had become subordinate to his will.

The mechanics of the corner

A successful market corner—though Hamanaka never explicitly called it that—requires three ingredients: physical control, financial leverage, and information asymmetry. Hamanaka had all three.

On the physical side, Sumitomo owned warehoused copper. When copper prices weakened, Hamanaka would bid aggressively to acquire more, signalling that Sumitomo was committed to absorbing supply. Smaller traders, worried that Sumitomo would corner available inventory and squeeze short sellers, would capitulate and buy ahead of prices rising. This created self-reinforcing upward momentum.

On the financial side, Hamanaka took massive long positions in copper futures and options. Unlike physical inventory, which had storage costs, futures cost little to hold. He could leverage Sumitomo’s balance sheet to maintain enormous notional copper exposure with relatively modest capital outlay. As prices rose—partly because of his own buying, partly because he was seen as holding key information—these positions appreciated. He could then liquidate selectively, capturing profits and redeploying capital into fresh accumulation.

The information advantage was crucial. Sumitomo knew its own positions. By maintaining opacity about the exact size of his long positions and physical holdings, Hamanaka created uncertainty in the market. Other traders didn’t know how much copper he held or when he might sell. This uncertainty depressed selling; nobody wanted to be short when Mr. Copper might squeeze them. Prices stayed elevated.

The long con, 1984–1995

For over a decade, the strategy seemed to work. Copper prices oscillated upward. Hamanaka booked trading profits that made him a star at Sumitomo. He was given a private office, answerable only to senior management in Tokyo. His compensation soared. Sumitomo’s shareholders celebrated the trading gains, unaware that they were funding an increasingly illiquid and leveraged bet on a single commodity.

But markets have gravity. In the early 1990s, recession weakened copper demand. Mines in Chile, Peru, and other producers increased output, adding supply. Prices softened. Hamanaka kept buying, convinced the price break was temporary and that his sheer conviction could bend prices back upward. He accumulated even more futures contracts and physical inventory. The losses began.

By 1995, Hamanaka was trapped. He had massive long positions that were underwater. He couldn’t liquidate without crashing prices and crystallising catastrophic losses. So he doubled down—a classic response to mounting losses in leveraged trading. He forged documents, falsified records, and misrepresented his position sizes to Sumitomo’s internal auditors and senior management. For a time, his fabrications worked. Tokyo accepted his assurances that the positions were sound and would reverse.

The collapse

In June 1996, Sumitomo’s auditors uncovered discrepancies in Hamanaka’s trade documentation. An investigation revealed that his actual losses were far larger than reported. The copper position couldn’t be hidden any longer. Sumitomo announced a $2.6 billion loss—at the time, one of the largest trading losses in history—and disclosed that Hamanaka had been engaging in unauthorised trading and falsifying records.

The announcement shocked markets. Global copper prices fell sharply as traders realised that the foundation of years of price strength—Mr. Copper’s relentless buying—had been a mirage. The LME suspended copper trading briefly to manage the dislocation. Other traders who had positioned for continued strength suddenly faced margin calls and forced liquidations.

Hamanaka was arrested. The Japanese authorities charged him with fraud and forgery. In 1998, he was convicted and sentenced to eight years in prison. He was released in 2005 after serving most of his sentence.

The lessons and fallout

The Hamanaka scandal exposed gaps in Sumitomo’s risk controls and in the oversight of commodity trading more broadly. A single trader, armed with corporate capital and minimal independent audit, had been able to hide losses and manipulate market prices for over a decade. He had defied basic risk-management principles: position limits, daily mark-to-market reconciliation, and separation of front-office trading from back-office verification.

The episode also revealed the fragility of commodity corners. A corner—the attempt to force buyers to pay extortionate prices by restricting supply—ultimately depends on the ability to sustain the position indefinitely. Once the bearer of the corner faces losses or pressure, the structure collapses. Hamanaka’s conviction that prices would rise and validate his positions was ultimately just that: a conviction. Markets care little for traders’ certainty.

For the LME and global commodity regulators, the scandal prompted tighter position-limit rules and more rigorous disclosure requirements. For Japanese corporate governance, it became a cautionary tale about the risks of opaque, concentrated trading authority. For Sumitomo itself, it was a humbling reminder that no single trader—however talented—should be trusted without institutional checks.

The copper market, after Hamanaka’s fall, remained vulnerable to price swings and supply shocks, but no single actor would ever dominate it so completely again.

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