The gonadotropin responses to a 4-hour infusion of gonadotropin-releasing hormone (GnRH), the prolactin (PRL) responses to a bolus injection of thyrotropin-releasing hormone (TRH), and seminal plasma dihydrotestosterone (DHT) levels were assessed before and 6 to 12 months after varicocelectomy was performed in 56 infertile men with varicoceles and sperm densities less than 30 X 10(6)/ml. The men were divided into four groups, determined by their sperm densities and hormonal parameters. Groups I (18 men) and II (12 men) had sperm densities less than 10 X 10(6)/ml, and groups III (16 men) and IV (10 men) had sperm densities of 11 to 30 X 10(6)/ml. The men from groups I and III had excessive preoperative gonadotropin and PRL responses, and lower-than-normal seminal plasma DHT levels. The men in groups II and IV had normal hormonal values. After operation, 12 of the men from group I and 11 from group II had improvements in seminal and hormonal parameters. The other men in these two groups and all of the men in groups II and IV had no changes in seminal and hormonal parameters after operation. This study indicates that an assessment of these hormonal parameters may be useful in predicting which men with varicoceles are likely to have an improvement in sperm density after varicocele repair.