Cyproterone acetate is a steroidal anti-androgen that has been used in human medicine for contraceptive purposes as well as treatment of medical conditions responsive to suppression of testosterone production. While serious side effects are considered to be rare, sporadic cases of severe hepatitis have been reported, including several fatal cases. This report describes a case of fatal fulminant hepatitis in one of four male chimpanzees (Pan troglodytes) that were undergoing trial treatment with cyproterone acetate to decrease inter-male aggression.