There is good evidence of an associated abnormality in testicular hormone production and spermatogenesis in some men with varicoceles. This abnormality can be demonstrated with dynamic tests of the hypothalamic-pituitary-testicular axis and by measuring seminal plasma androgen levels. A high proportion of oligozoospermic men who have abnormal hormone profiles will respond favorably to correction of their varicosities. Several oligozoospermic men with varicoceles have normal hormonal profiles. To date, in our unit, none of these men has had an improvement in seminal characteristics after varicocelectomy. This result would suggest that these men have incidental varicoceles. It is not clear what the testicular defect is leading to abnormal spermatogenesis in these men. Clearly, more studies are required in this group of men and in the men with sperm densities greater than 30 X 10(6)/ml, the majority of whom have normal responses to GnRH infusion. More information is needed regarding the intratesticular control of hormone production and spermatogenesis. As our knowledge of the paracrine system within the testis increases, so should our understanding of the mechanisms involved in the association of varicoceles and infertility.