The fertility of 134 bipolar probands (60 males, 74 females) was compared with national census data and examined for sex and age-of-onset effects. Fertility was markedly reduced in certain age cohorts in both females and males. When illness onset was taken into account, significant reductions in fertility were observed in the two sexes both before and after onset. The overall reduction in females remained constant over time, but was greater after onset among males. The role of selective forces maintaining affective disorders in the population was discussed in light of genetic and sociodemographic hypotheses.