The serum concentrations of prolactin (PRL), follicle stimulating hormone (FSH) and luteinizing hormone (LH) and the plasma concentrations of testosterone, 5-alpha-dihydrotestosterone (DHT) and oestradiol were measured in 80 infertile men and 38 men of known fertility. The infertile men had a lower mean concentration of prolactin and a higher mean concentration of FSH than the fertile controls. The mean FSH was particularly high in infertile men without antisperm antibodies but with oligospermia (sperm density up to 20 million/ml), who also had a higher mean serum LH level than the fertile controls. Both FSH and LH showed an inverse relationship with sperm density. There were no significant differences in plasma testosterone, DHT or oestradiol between fertile and infertile men. In both the fertile and infertile groups strong positive associations were found between the concentrations of FSH and LH, and between testosterone and DHT. Weaker relationships were found between DHT and oestradiol, and between testosterone and PRL. In the fertile men, positive associations were also found between LH and testosterone, and between oestradiol and both FSH and LH. Testosterone and oestradiol were associated only in infertile men. The relevance of these findings to the aetiology of male infertility is discussed.