LAS VEGAS -- During a year-long gambling binge at the Caesars Palace and Rio casinos in 2007, Terrance Watanabe managed to lose nearly $127 million.

The run is believed to be one of the biggest losing streaks by an individual in Las Vegas history. It devoured much of Mr. Watanabe's personal fortune, he says, which he built up over more than two decades running his family's party-favor import business in Omaha, Neb. It also benefitted the two casinos' parent company, Harrah's Entertainment Inc., which derived about 5.6% of its Las Vegas gambling revenue from Mr. Watanabe that year.

Today, Mr. Watanabe and Harrah's are fighting over another issue: whether the casino company bears some of the responsibility for his losses.

In a civil suit filed in Clark County District Court last month, Mr. Watanabe, 52 years old, says casino staff routinely plied him with liquor and pain medication as part of a systematic plan to keep him gambling.

Nevada's Gaming Control Board has opened a separate investigation into whether Harrah's violated gambling regulations, based on allegations made by Mr. Watanabe.