BACKGROUND: The impact of pregnancy on Crohn's disease activity has been poorly investigated. AIM: To determine the effect of pregnancy on Crohn's disease activity from the retrospective analysis of a cohort of women who had a regular clinical follow-up. METHODS: Seventy pregnancies occurring in 61 women were studied. The Harvey-Bradshaw index was determined during the four quarters preceding each pregnancy, the three quarters of pregnancy and the four quarters following delivery. RESULTS: The mean Harvey-Bradshaw index during pregnancy [0.68 (0.18), mean (S.E.M.)] was significantly lower than that of the year preceding pregnancy [0.98 (0.16), P = 0.03] and that of the year following delivery [1.10 (0.17), P = 0.04]. In non-smoking women (48 pregnancies), there was no significant change of Harvey-Bradshaw index between these intervals. Whereas in those who smoked (22 pregnancies), most of whom reduced tobacco consumption during pregnancy, the mean Harvey-Bradshaw index during pregnancy was significantly reduced compared with that of the year following delivery [0.58 (0.20) vs. 1.60 (0.33), P = 0.01]. The use of drugs was significantly lower during pregnancy. CONCLUSIONS: Crohn's disease activity is mildly but significantly lower during pregnancy. The reduction of tobacco consumption during pregnancy in smoking women may play an important role in this improvement.