A systematic survey of 1599 male patients attending a subfertility clinic has shown that 2.2% were chromosomally abnormal. This frequency was approximately five times higher than that found among the normal male population. The majority of chromosome abnormalities (at least in chromatin-negative males) appeared to be exerting their effect on male fertility through disturbance of spermatogenesis. The contribution made by chromosome abnormalities causing recurrent abortion in wives was negligible. The possible reasons underlying impairment of spermatogenesis in man are discussed in relation to the findings in other species, and the suggestion that some elimination of chromosomally unbalanced gametes occurs from the human male germ line is proposed.