OBJECTIVE To study the effects of varicocele on testicular function in adolescents. METHODS A prospective controlled study in 88 randomly selected adolescents. METHODS All participants were referred to the fertility outpatient clinic of our University Hospital. METHODS All participants with a grade II varicocele (group 2) or a grade III varicocele (group 3) were selected at a district military medical council, whereas a similar group of healthy volunteers without a varicocele served as controls (group 3). METHODS Testis volumes were measured using an orchiometer. Semen analysis was performed according to standard procedures, and serum hormone levels were determined using a radioimmunoassay. METHODS Testis volumes, semen quality, and hormonal parameters in adolescents with and without a varicocele were compared. RESULTS In group 1 (n = 21), the mean left testis volume (24.5 mL; 95% confidence interval [CI]: 22.8 to 26.2) was significantly (P less than 0.05) different from group 2 (n = 15) (20.9 mL: 95% CI: 18.5 to 23.4) and group 3 (n = 52) (20.7 mL; 95% CI: 19.2 to 22.2) (P less than 0.01) adolescents. In adolescents with a pronounced varicocele-associated left testicular growth failure, the total sperm number was reduced. However, sperm concentration, motility, and morphology were not altered. Luteinizing hormone, follicle-stimulating hormone, testosterone, and prolactin levels were all within the normal ranges in the three groups. CONCLUSIONS Left testicular growth failure in adolescents with a varicocele is only associated with a decrease in total sperm number.