A federal judge's ruling this week that the entry fees paid to play on the fantasy horse racing website Derby Wars are wagers—the first time a U.S. court has taken that position—could be a warning signal to the booming fantasy sports industry, lawyers say.

The decision was issued by Judge James Otero of the U.S. District Court for the Central District of California in a case pitting Louisville, Kentucky-based Horse Racing Labs, the parent company of Derby Wars, against The Stronach Group, a collection of horseracing associations operating at tracks including Santa Anita Park in Southern California, Golden Gate Fields in San Francisco and Pimlico in Baltimore, Maryland.

“The Derby Wars decision could tip the scales against fantasy sports operators in several other lawsuits, especially those in Illinois, Massachusetts, Texas and New York, where the courts are confronted with the similar question of whether fantasy sites constitute a form of gambling or betting,” according to Florida-based attorney Daniel Wallach of Becker & Poliakoff, an authority on sports, fantasy sites and gaming.