Previous studies have suggested that chrysin (5,7-dihydroxyflavone) is a potent aromatase inhibitor in vitro, which may increase serum testosterone concentration. Testosterone is a highly anabolic hormone which can stimulate the growth of muscle mass and strength. However, the effects of the chrysin supplementation on body composition, muscle strength, and exercise performance in human have not yet to be discussed. PURPOSE: To investigate whether the chrysin supplementation would improve body composition, muscle strength, and exercise performance by altering the synthesis of testosterone. METHODS: Twenty throwing athletes (14 males, 6 females) of National Taiwan Sport University participated in this study. Subjects were randomly assigned in pairs (matched for athletic events) by use of a double blind design to receive chrysin (group C) or placebo (group P). All subjects were given oral chrysin doses of 3 g or placebo per day for 21 days. Body composition, muscle strength and exercise performance were also assessed at day 1, 8, and 22 in the morning. Body composition was evaluated by body weight, body mass index (BMI), fat mass, fat free mass (FFM), body fat percentage, skinfold thickness, and body circumference. Muscle strength and exercise performance including maximal isometric force of quadriceps and biceps, vertical jump, standing broad jump, and medicine ball chest pass test were performed. A two-way mixed design analysis of variance (ANOVA) with one within-subject factor (time trial), one between-subject factor (group C and P), and their interaction were used to determine the overall 21-day treatment effect and level of significance. All data were expressed as mean ± SD and values of P<0.05 were considered significant. RESULTS: BMI was reduced in day 8 and 22 at group C (32.1  4.9 vs. 31.9  4.8 kg/m2; P<0.05). FFM was increased in day 8 at group P (72.2  13.4 vs. 73.1  13.0 kg; P<0.05). Waist circumference was reduced in day 22 at group C (97.1  11.4 vs. 95.4  10.6 cm; P<0.05). The data showed no significant difference in body composition, muscle strength, and exercise performance between groups C and P. CONCLUSION: Our data provided by a human study, suggest that chrysin supplementation for 21 days may not improve body composition, muscle strength, and exercise performance.