PURPOSE: We evaluated the effect of patient age at treatment of cryptorchidism in relation to subsequent semen quality. MATERIALS AND METHODS: Semen analyses and hormonal evaluations were performed in 51 men who were treated for cryptorchidism at ages 10 months to 12 years. RESULTS: Sperm concentration was normal in 90% of the patients with unilateral and 50% with bilateral cryptorchidism. No patient treated before age 4 years had severe sperm defects. Elevated follicle-stimulating hormone levels indicated severe testicular damage. CONCLUSIONS: Fertility is better in patients with bilateral cryptorchidism if treated before age 4 years. Age at treatment did not have a significant effect on semen quality in patients with unilateral cryptorchidism.