A 34-year-old male patient was studied because of primary infertility. He was a normally virilized man with normal potency, habitus, and sense of smell. The testes were of normal consistency and volume. Semen analysis showed azoospermia. The baseline serum concentrations of luteinizing hormone (LH) were normal, but those of follicle-stimulating hormone (FSH) were undetectable. After stimulation with gonadotrophin-releasing hormone (GnRH), 100 micrograms, iv, the response of LH was normal, whereas the rise in FSH levels was minimal. The administration of clomiphene citrate (100 mg/day for 21 days) increased LH levels, but FSH levels remained low. Serum concentrations of testosterone and estradiol were normal. Baseline serum levels of thyrotrophin, prolactin, growth hormone, corticotrophin and cortisol were also normal. The patient refused testicular biopsy and treatment with purified human FSH. Our results suggest a possible defect for FSH production at the pituitary level.