A total of 843 consecutive boys (median age 12.7 years) who had undergone testicular biopsy at operation for undescended testis was followed into adulthood (median age 25.2 years) to examine for testicular germ cell neoplasia. Five cases of testicular germ cell neoplasia were identified, including 1 nonseminoma of the contralateral testis, which had been treated before surgery for an undescended testis, 1 nonseminoma found at followup, 1 seminoma and 2 intratubular germ cell neoplasms. Of the later 3 patients 1 had a 45,X/46,XY karyotype and 2 had abnormal external genitalia. Previous testicular biopsy from the patient in whom nonseminoma was noted at followup showed Sertoli cells only. We recommend that testicular biopsy be performed at operation for undescended testis in boys with abnormal sex chromosomes, particularly 45,X/46,XY karyotype, and in those with abnormal external genitalia. The pattern of Sertoli cells only in biopsies from boys does not preclude the occurrence of testicular cancer in adulthood.