New York Yankees third baseman Alex Rodriguez tested positive for stimulants in 2006, according to the New York Times.
Rodriguez was not publicly identified for failing the test because players face suspensions for banned substances only if they test positive more than once.
Rodriguez is appealing a 211-game suspension for violating Major League Baseball's drug policy.
Lanny J. Davis, a representative of Rodriguez's legal defense, denied that Rodriguez failed a drug test seven years ago. In a statement, he wrote that Major League Baseball is acting unethically.
"The ethically questionable and possibly illegal misconduct of Major League Baseball in its investigation of Alex Rodriguez -- such as the knowing purchase of stolen documents for $125,000 in cash in a satchel in a Florida restaurant and putting in a good word with prosecutors for someone reportedly under federal and state investigation for distributing drugs to teenagers in the name of getting Alex Rodriguez -- is not just unseemly, it is shameful.
"I believe a federal investigation of this misconduct is needed -- and specifically, of the commissioner of baseball and the extent of his involvement and knowledge of the professional misconduct by investigators he hired, as reported by The New York Times."
It is not known if the league used the failed drug test as part of its evidence against Rodriguez during recent appeal hearings.
Rodriguez's appeal is on hiatus. MLB already argued its case. Rodriguez will present his defense when the hearings resume Nov. 18.
Rodriguez is also suing commissioner Bud Selig and MLB, claiming misconduct. His lawsuit will be heard beginning Nov. 7. The league is trying to have the suit dismissed.