Two men with serum levels of follicle-stimulating hormone (FSH) persistently below 3 mIU/ml and normal levels of luteinizing hormone (LH), thyroid-stimulating hormone (TSH), growth hormone, prolactin, cortisol, and testosterone are reported. The intravenous administration of thyrotropin-releasing factor led to a normal increase in TSH and prolactin levels. Gonadotropin-releasing factor stimulation resulted in a net increase of 2 mIU/ml and 25 mIU/ml for FSH and LH, respectively. The administration of clomiphene resulted in a normal FSH increase in both patients, an LH increase in one, and a serum testosterone increase in the other. These results suggest a possible defect for FSH production at a level above the pituitary. Semen analyses revealed abnormalities in motility and morphology. A testicular biopsy from one patient revealed delayed maturation of spermatogenesis. It is recommended that serum FSH determinations be included in studies of male infertility patients.