The incidence of relapses of inflammatory bowel disease in pregnancy ranges from 20 to 35%. The effect of pregnancy on Crohn's disease and ulcerative colitis after pregnancy has been investigated here. During the survey period, 29 pregnancies occurred in 18 Crohn's patients, and 25 in 19 ulcerative colitis patients. Incidence of relapses during pregnancy and post-partum was 14 and 17% in Crohn's, and 36 and 12% in ulcerative colitis. Fourteen Crohn's patients (17 pregnancies) and 17 ulcerative colitis patients (19 pregnancies) were followed for 3 years before pregnancy and for 4 years after delivery. During the 3 years after pregnancy, the number of relapses/year was significantly lower compared to the 3 years before pregnancy and to the incidence in controls, both in Crohn's (p < 0.05) and ulcerative colitis (p < 0.005). Changes in nutritional status were recorded in 41% of Crohn's, but in none of the ulcerative colitis patients. The reduced incidence of relapses following delivery in Crohn's patients was more marked in the 10 with normal nutritional status at the time of conception (70% reduction in number of relapses, p < 0.05), while in the 7 significantly underweight patients, the reduction was slight (27% reduction in number of relapses, p = NS). In conclusion, the incidence of relapses in the first 3 years after pregnancy is lower than in the pre-pregnancy period. In Crohn's disease, the effect is more evident in patients with normal nutritional status at the time of conception than in malnourished patients.