To determine the effects of anabolic steroids on myocardial structure, VO2max, and body composition, experienced age-matched male weight trainers (M age 26.5 yr) who either used (U) (N = 11) or did not use (NU) (N = 13) anabolic steroids were evaluated. Steroid users were tested while off cycle (U-OFF) for at least 8 wk, again at the peak (U-ON) of their subsequent cycle, and to the nonuser group of weight trainers. Echocardiographic measurements revealed significant differences in left ventricular (LV) mass (182.8 +/- 26.9 g vs 210.6 +/- 42 g; P < 0.05) and interventricular septum thickness (IVS) (10.3 +/- 1.2 mm vs 11.1 +/- 1.2 mm; P < 0.05) between U-OFF and U-ON, respectively. NU measurements were also significantly different than U-ON for LV mass and IVS (186.5 +/- 36.2 g; P < 0.05 and 9.3 +/- 1.2 mm; P < 0.05, respectively). LV diameter in diastole was significantly greater in U-ON (59.1 mm) than in NU (55.7 mm; P < 0.05). In addition, LV posterior wall thickness in diastole was greater in U-ON compared with NU (11.2 mm vs 9.5 mm; P < 0.05). VO2max values for both user groups were significantly lower than those for NU (U-OFF = 41.0 +/- 4.5 ml.kg-1.min-1, U-ON = 41.0 +/- 5.7 ml.kg-1.min-1, and NU = 50.2 +/- 6.4 ml.kg-1.min-1; P < 0.05). Despite these morphological changes within the myocardium, there were no concomitant increases in shortening fraction.