Delayed orchidopexy for undescended testes has been associated with abnormal testicular histology, but the effect on later fertility is unknown. We aimed to establish the importance of the first postnatal maturational step-ie, the transformation of gonocytes into Ad (dark) spermatogonia on fertility. We matched histological findings of the testes from 31 patients who had undergone an early orchidopexy with their total number of sperm. If Ad spermatogonia were present, 17 (94%) of 18 (95% CI 72.7-99.9) of the men had a total sperm count of 40 x 10(6)/ejaculate or greater. By contrast, despite successful early surgery, if Ad spermatogonia were absent, 12 (92%) of 13, (64.0-99.8) patients had abnormal spermiograms. Thus, the transformation of gonocytes into Ad spermatogonia is crucial for male fertility.