Sperm analysis and serum hormone measurements (LH, FSH, testosterone) were performed in 29 patients after orchiectomy for seminoma before and after irradiation. Before radiotherapy 14 of 20 orchiectomized patients were azoospermic or had impaired spermatogenesis. A minimum sperm count was found 1 year after radiotherapy with gradual improvement up to 2 years. The recovery of sperm cell production was impaired most in patients with pre-treatment sperm counts less than 3 million/ml. Serum testosterone remained at low normal levels throughout the observation period. The mean serum FSH was increased 1 year after radiotherapy but returned to normal in 50% of patients within 3 years after treatment. This post-treatment increase in FSH was significantly correlated with increased pre-treatment FSH but not with the gonadal dose, which was 1 to 3% of the target dose. Severe disturbances in spermatogenesis, observed 2 to 3 years after radiotherapy for early seminoma, are likely to be the expression of a highly impaired pre-treatment sperm cell production and only to a lesser degree dependent on the irradiation of the remaining testicle.