Meiotic and histological studies have been carried out on testicular biopsies from an azoospermic human male who exhibited normal secondary sex characteristics. A failure in the formation of chiasmata during the first meiotic prophase resulted in a breakdown of the subsequent meiotic and spermatogenic processes. The few spermatozoa formed exhibited a range of DNA contents consistent with a random orientation and movement of the meiotic chromosomes. Radiation studies carried out on peripheral lymphocytes demonstrated that the patient had a reduced facility for chromosomal DNA repair. It is suggested that this defect gives an explanation for the failure of chiasma formation since chiasma formation probably involves a breakage, crossingover and repair of chromosomal DNA during the first meiotic prophase.