From a computer file of 955 couples attending hospital because of an infertile marriage, 279 were identified where the female partner had normal investigation results. Follow-up information was available for all couples and the fecundability (conception rate per month) for the first year of follow-up has been calculated, stratified according to the husband's investigation results. The fecundability of couples where the wife had normal investigation results but the husband had reduced sperm density ranged from 0.4% for wives with azoospermic husbands to 5.9% when the husband had at least 10 million motile sperm/ml. For any given number of motile sperm the fecundability was also strongly dependent on the duration of involuntary infertility as recorded at the time of initial clinic attendance. In view of the lack of effective treatment for most male infertility the fecundability rates reported herein are likely to represent those that would be found in an untreated population and should be useful as a base line when evaluating results of new treatments.