Little is known about the biological and clinical significance of bilaterally enlarged testes (bilateral megalotestes or macroorchidism). We have investigated clinical, endocrine and spermatological characteristics of 135 andrological patients with a testicular volume of more than 25 ml bilaterally. 135 patients with normal testicular volumes served as controls. Subjects with bilateral megalotestis had higher sperm counts and better sperm morphology and motility than the controls. Levels of LH, FSH, prolactin, testosterone and estradiol were lower in the megalotestis group. This was statistically significant for LH and FSH. No testicular tumours were diagnosed in either group. We conclude that in most andrological patients with bilateral megalotestes fertility parameters are excellent despite the unusually low hormone levels found in a significant minority. No specific pathology underlying the large gonadal volume could be identified.