Purpose: The aim of this study was to find out the relationship in the patient with cryptorchidism between the number of spermatogonia at surgical age and spermiogram data at adulthood. Methods: Between 1971 and 1996 the authors surgically treated 1,550 children for 2,249 undescended testis. in 21 unilateral and 41 bilateral cryptorchidism patients, both testis were biopsied at surgical procedure during childhood. We currently have spermiogram data for a patient at adulthood. Results: By using the best Tubular Fertility Index (TFI) figures of both testis, the Sperman correlation index between the two parameters is r = 0.22, statistically nonsignificant. The TFI test sensitivity is 0.68 and the specificity is 0.60. The likehood ratio for a positive test result is 0.84 and the likehood ratio for a negative test result is 0.37. Conclusion: Although the authors have only one spermiogram, and there are few cases, they suspect the TFI is not a good index to predict the potential fertility in cryptorchidism patients. Copyright (C) 1998 by W.B. Saunders Company.