Six male strength athletes, three illicit anabolic steroid users and three non-steroid users were monitored over several months as they underwent normal training and competition routines. Subjects completed the Profile of Mood States (POMS) questionnaire, Buss-Durke Hostility Inventory and the Rosenweig Picture Frustration Test on four occasions: two on-drug periods and two off-drug periods for the steroid users and equivalent test periods for the non-users. Steroid presence was monitored objectively by gas chromatography coupled to mass spectrometry. Whilst those drugs declared by the athletes were confirmed in the “on-drug” samples and most of the “off-drug” samples, clear steroid traces were apparent in some supposed off-drug sessions. This complicated data interpretation and may partially explain why steroid users were significantly more hostile and aggressive at all times compared to the non-users. Despite this, self-rated aggression did increase significantly in steroid users during their acknowledged on-drug periods. Multiple drug use or “stacking” in particular caused severe hostility/aggression; one steroid user also admitted to attempted murder during a previous steroid-taking phase.