Endogenous testosterone levels have been linked to aggressive behaviour in both animals and humans. Studies administering moderate doses of exogenous testosterone for contraceptive and clinical purposes reveal essentially no adverse effects on male sexual and aggressive behaviour. However, investigations and case reports of athletes, usually involving higher doses, demonstrate an association between anabolic-androgenic steroid use and affective and psychotic syndromes and psychological dependence. Efforts to study the psychological and behavioural effects of anabolic-androgenic steroids are complicated by a variety of methodological limitations. Only 3 prospective, blinded studies demonstrating aggression or adverse overt behaviour resulting from anabolic-androgenic steroid use have been reported. With estimates of over 1 million past or current users in the US, an extremely small percentage of individuals using anabolic-androgenic steroids appear to experience mental disturbances severe enough to result in clinical treatment and medical case reports. Even among those so affected, the roles of previous psychiatric history, genetic susceptibility to addictions or mental disorders, environmental and peer influences, and individual expectations remain unclear.