The recovery of exocrine and endocrine testicular function was studied in six patients orchidectomized for an estrogen-producing Leydig cell tumor. Gynecomastia disappeared in four patients. The contralateral testis, whose volume was reduced, returned to normal size after 30 days. Sperm density returned to normal in only one of the four patients in whom the preoperative sperm count was reduced. One day after unilateral orchidectomy, plasma estradiol decreased to normal and testosterone (T) fell about 50%. On the 10th postoperative day, plasma T [5.60 +/- 1.20 ng/ml (SD)] was normal. On day 120, T was higher than on day 10 (6.83 +/- 1.20 ng/ml). There was no significant increase of T after a single injection of hCG (5000 IU) on day 10, and the T response was similar to that of normal men on day 120. Plasma FSH and LH were increased on the 10th postoperative day; they then decreased between 60 and 120 days after the operation but were still above the normal values on day 120. The FSH/LH ratio, which was 0.43 +/- 0.17 preoperatively returned to normal (1.60 +/- 0.25) 10 days postoperatively. In conclusion, after hemicastration for an estrogen-secreting tumor, testicular hormonal secretion returns to normal within 120 days but spermatogenesis may still be impaired at this time.