Cervical intraepithelial neoplasia (CIN) has its highest incidence during women's reproductive years. During 2 sequential 7-year periods, 1994 to 2000 and 2001 to 2007, 3695 and 3894 deliveries were performed, respectively, at Osaka University Hospital. CIN was detected in 21 cases (0.57%) during 1994-2000 and in 43 cases (1.1%) during 2001-2007. By comparison, cervical intraepithelial neoplasia-complicated pregnancies increased significantly in the latter period (P = .015 by Fisher exact test, Odds ratio = 1.95; 95%CI: 1.16-3.30). We observed CIN regression in 34 (76%) of 45 cases of vaginal delivery and in 6 (50%) of 12 cases of cesarean delivery, indicating that the outcome of an initially diagnosed CIN and the delivery routes appeared not to be significantly related. However, a different result was obtained when only those patients whose CIN lesions persisted until the delivery were analyzed. Among the 35 such cases in the vaginal delivery group, 24 cases (69%) regressed after the delivery; in 8 such cases from the cesarean delivery group, only 2 cases (25%) regressed afterward. Our study clearly shows that pregnancy complicated with CIN is increasing rapidly in Japan. We also find that there is a significantly more frequent postpartum regression of biopsy-proven CIN lesions following a vaginal delivery compared to cesarean section (P = .042 by Fisher exact test, Odds ratio = 6.55; 95% CI: 1.13-37.8).