The gonadal function of 18 patients with testicular germ cell tumors was evaluated. Seminal parameters after orchiectomy were examined in 15 patients. Six of them were available for follow-up observation after 2 or 3 courses of adjuvant chemotherapy. Serum gonadal hormones before and after orchiectomy were evaluated in 7 patients (testosterone and PRL were not examined in one patient). Five of 15 (33.3%), 8 of 15 (53.3%), 13 of 15 (86.7%), 7 of 13 (53.8%), and 9 of 12 (75.0%) had abnormal values in seminal volume, sperm concentration, motility, morphology, and vitality, respectively. The sperm concentration gradually improved after chemotherapy following orchiectomy in 5 of 6 (83.3%) patients. In all the patients examined, serum levels of follicular stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL) increased after orchiectomy. Serum levels of testosterone increased in 4 patients, but decreased in 2 after orchiectomy. These findings suggest that several factors, including preexisting intrinsic defect and disturbance of the hypothalamus-pituitary-gonadal axis, are involved in the deterioration of gonadal function in patients with testicular germ cell tumors.