Cryptorchidism is one of the testicular factors recognized as preventable disease processes. Although the effect of bilateral cryptorchidism on fertility is well documented, more studies are required in order to determine the bilateral functional and histologic effects of unilateral cryptorchidism. This clinical retrospective study analyses the effects of cryptorchidism on fertility and histopathologic changes in dystopic and eutopic testes. Among 1400 patients consulted in our infertility clinic between the years of 1989 and 1993, 47 patients (3.3%) had cryptorchidism (29 unilateral/18 bilateral). Among these patients 26 had a history of orchidopexy (mean age at the time of operation: 13.11 +/- 5.24 years). Semen analyses (3x) were available in 43 and hormone profile in 27 out of 47. The incidence of abnormal semen parameters in unilateral and bilateral group was 89.6% and 100%, respectively. Testicular biopsies were obtained from 17 patients (10 with unilateral and 7 with bilateral disease). The biopsy findings in 75% of eutopic testes in unilateral group were similar to that of dystopic testis. The high incidence and significant histologic changes in eutopic testis supports the hypothesis that the disease might be bilateral in nature. The question whether the progression of the disease can be stopped by early orchidopexy still remains unanswered until long term follow-up studies are completed.