Flow cytometric analysis was performed on the testicular aspirates of 45 consecutive children with unilateral cryptorchidism undergoing elective orchiopexy or orchiectomy. Concomitant histological analysis was performed on the testicular tissue obtained from either biopsy or orchiectomy specimens. In all cases deoxyribonucleic acid histograms appeared to correspond with microscopic appearance. Histograms from prepubescent patients demonstrated 85 to 95% of cells in the diploid (2c) peak and less than 10% of cells in the tetraploid peak (4c), representing prepubertal testes without active spermatogenesis. Three distinct patterns of ploidy were identified in postpubertal children corresponding to the histological appearances of normal spermatogenesis, maturation arrest and the Sertoli-cell-only syndrome, respectively. In addition, we identified an aneuploid cell population in the specimen from 1 patient, suggesting that this testis may be at risk for future malignant degeneration. We conclude that flow cytometry of testicular aspirates is an easy and effective means of testicular evaluation, which may permit predictions regarding the fertility and malignant potential of undescended testes in postpubertal children.