In a nationwide epidemiological study, 2,506 men and 5,296 women with multiple sclerosis (MS) completed a questionnaire concerning disease course and reproductive history. The mean number of children was the same for both men and women (1.7). The sex ratio for offspring of male probands was 0.99; it was significantly lower than that of offspring of female probands (1.07). As the sex ratio of French births is 1.05, the observed difference seemed due to an altered sex ratio in fathers with MS. This atypical sex ratio of births for male patients was examined in relation to MS course. We found that it was mainly due to an excess of female births after the onset of the disease. Altered sex ratio and MS might have some risk factors in common.