Although the aetiology of Crohn's disease is unknown, morphological and epidemiological studies have implicated measles virus as a potential component cause, particularly when exposure occurs in utero or early in life. An increased incidence of Crohn's disease among people born during measles epidemics would support this hypothesis. We identified all individuals born in four counties in central Sweden in 1945-54 who had had Crohn's disease diagnosed before the age of 30 years. Yearly reports compiled in these counties revealed that five measles epidemics had affected all four counties during the trial period. After adjusting for monthly differences in the number of livebirths in the four counties, we calculated the expected number of patients with Crohn's disease and ulcerative colitis born during the 3-month period after the peaks of the epidemics. The number of people with Crohn's disease significantly exceeded that expected: 57 versus 39.0 (standardised incidence ratio 1.46, 95% CI 1.11-1.89). For patients with ulcerative colitis, the observed number (42) was close to that expected (46.8). Our results strengthen the hypothesis that measles is related to Crohn's disease and that the perinatal period is a time of vulnerability.