Plasma follicle stimulating hormone, testosterone and urine 5-hydroxyindole acetic acid levels were measured in 300 patients seen at an andrology clinic. Testicular biopsies were obtained from many of the severely oligospermic or azoospermic patients. Plasma follicle stimulating hormone concentrations were related inversely to sperm cell concentration, while plasma testosterone concentrations were depressed as the sperm count decreased to 5 times 106 per ml. The urine levels of 5-hydroxyindole acetic acid followed the same pattern as follicle stimulating hormone and were elevated in the group of individuals with sperm counts less than 10 times 106 per ml. Elevations in 5-hydroxyindole acetic acid often were associated with abnormalities in the testicular biopsy. Only 30 per cent of the patients who had elevated 5-hydroxyindole acetic acid also had elevated follicle stimulating hormone or depressed testosterone levels. The factors responsible for the elevation in 5-hydroxyindole acetic acid levels as the sperm count declines remain unclear.