Previous reports concerning isolated follicle stimulating hormone (FSH) deficiency and its possible pathogenesis have been conflicting. Both "normal" and "abnormal" FSH response to luteinizing hormone releasing hormone (LHRH) infusion have been described. We studied a 22-year-old man with normal basal serum testosterone and luteinizing hormone (LH) levels but undetectable levels of serum FSH. His serum LH titers showed one secretory spike during a 40-hour sampling at 20-minute intervals, whereas his serum FSH titers remained undetectable (less than 0.4 IU/l). Infusion of LHRH, 0.2 microgram/minute for 4 hours, induced the expected rise in the serum LH levels, but serum FSH levels remained low and only at one point reached 0.9 IU/l (normal adult male basal range 0.9-10.3 IU/l). The patient received LHRH, 100 micrograms/day, for three days. A second LHRH infusion, 0.2 microgram/minute for 4 hours, induced a normal rise in both the serum LH and FSH titers. The serum sex steroid binding globulin level was 10.3 ng DHT bound/ml (normal adult male level 8.0 +/- 0.3 ng DHT bound/ml). Presence of circulating auto-antibodies to the serum FSH was excluded by determining the binding of [125I] FSH with the patient's serum and comparing it with sera obtained from two normal male adult volunteers. Pituitary function tests were otherwise intact. Presence of a pituitary tumor was excluded by computerized axial tomography and x-ray studies of the pituitary fossa and normal visual fields. Clinically, the patient demonstrated cryptorchidism, hypospadias, surgically repaired omphalocele, and bilateral hearing loss.(ABSTRACT TRUNCATED AT 250 WORDS)