We report on 226 male patients with cryptorchidism 6 months to 16 years old who underwent open testis biopsy at orchiopexy or orchiectomy at Children's Hospital and Health Center from 1986 to 1990. A total of 355 specimens was obtained. Tissues were preserved in Bouin's solution and examined on light microscopy for fertility index measurements. Several biopsies prepared using Bouin's preserved paraffinized tissue and glutaraldehyde preserved semi-thin cut tissue were found to have comparable fertility index measurements. Of 184 patients with unilateral undescended testes 87 also underwent biopsy on the contralateral descended testis. A total of 42 patients had bilateral undescended testes. Age matched comparisons were made between fertility index measurements of the undescended testes and those previously reported of normal testes. Additional case matched comparisons of fertility indexes were made in those children who underwent biopsy of the undescended testis and its descended mate. Statistical analysis was performed using the independent Student t-test. When comparing undescended to descended testes, there was no significant difference in the fertility index of patients 1 year old or younger but fertility index differences were statistically significant in all of the other age groups. Fertility index measurements were significantly decreased from normal expected values in all age groups with unilateral cryptorchidism and in all but the 13 to 18-month-old group with bilateral cryptorchidism. The fertility index in the descended testis was similar to previously reported normal testis measurements in boys between 13 months and 6 years old. Our data suggest that potential fertility in the cryptorchid testis may be significantly impaired compared to normal testicular fertility regardless of patient age at the time of discovery of the undescended testis. The fertility index of the descended mates of unilateral undescended testes may also be somewhat impaired in certain age groups. Orchiopexy in the first year of life may be indicate to preserve available fertility potential.