No epidemiological data on inflammatory bowel disease (IBD) are available in France. We therefore conducted a prospective epidemiologic study of IBD in the Nord-Pas de Calais region and the Somme department of France (4.5 million inhabitants). Each suspected new case was reported by all (private and public) gastroenterologists (n = 120) and a questionnaire was filled out at the gastroenterologist office by an epidemiologist. The final diagnosis of Crohn's disease (CD), ulcerative colitis (UC), or proctitis (UP) was made in a blind manner by two gastroenterologists. During 1988, 576 IBD patients were identified; 281 (49 percent) had CD, 207 (36 percent) had UC including 75 UP; and 88 (15 percent) had unclassified colitis. The incidence rate per 10(5) was 6.3 for CD and 4.6 for UC. The female/male ratio was 1.4 for CD and 0.9 for UC. The mean age at the time of diagnosis was 31 years for CD and 40.5 years for UC. The mean time between onset of symptoms and diagnosis was longer for CD (15 months) than for UC (6.8 months). These preliminary data suggest that the incidence of IBD is high in Northwestern France and comparable, for CD, to the highest incidence of Northern Europe.