Increased muscle mass in subjects using AAS has been proposed to result from muscle hypertrophy alone [12] or from both muscle hypertrophy and hyperplasia [38]. Muscle hypertrophy is often evident by increased muscle fiber size and increased number of myonuclei. The latter is associated with satellite cell activation and myoblast infusion with the existing muscle fibers, leading to greater numbers of myonuclei in larger myofibers [39]. In previous studies on subjects with long term AAS supplementation (9±3.3 years), we observed significant higher frequency of newly formed myofibers in AAS users than in the non-AAS users, indicating that steroid can induce both muscle hypertrophy and hyperplasia [14], [15]. In the present study, long term AAS supplementation was only associated with higher lean leg mass, but not with larger fiber size, indicating that muscle fiber hyperplasia may play a role in the muscle mass enhancement. Coincidently, the number of myonuclei in type I fibers in the doped athletes was significantly higher than in the clean athletes, which may indicate satellite cell activation for muscle fiber hyperplasia.