The purpose of this study was to examine the effect of isokinetic eccentric (ECC) and concentric (CON) training at two velocities [fast, 180 degrees s(-1 )(3.14 rad s(-1)) and slow,30 degrees s(-1)(0.52 rad s(-1))] on muscle hypertrophy. Twenty-four untrained volunteers (age 18-36 years) participated in fast- ( n=13) or slow- ( n=11) velocity training, where they trained one arm eccentrically for 8 weeks followed by CON training of the opposite arm for 8 weeks. Ten subjects served as controls (CNT). Subjects were tested before and after training for elbow flexor muscle thickness by sonography and isokinetic strength (Biodex). Overall, ECC training resulted in greater hypertrophy than CON training (P<0.01). No significant strength or hypertrophy changes occurred in the CNT group. ECC (180 degrees s(-1)) training resulted in greater hypertrophy than CON (180 degrees s(-1)) training and CON (30 degrees s(-1)) training (P<0.01). ECC (30 degrees s(-1)) training resulted in greater hypertrophy than CON (180 degrees s(-1)) training (P<0.05), but not CON (30 degrees s(-1)) training. ECC (180 degrees s(-1)) training resulted in the greatest increases in strength (P<0.01). We conclude that ECC fast training is the most effective for muscle hypertrophy and strength gain.