Inflammatory bowel disease: one condition or two?

Demographic data obtained from epidemiological studies of Crohn's disease and ulcerative proctocolitis in Regio Leiden (Leiden health care region) were compared. Ulcerative colitis (6.8 cases/10(5) population/year) had an incidence almost twice that of Crohn's disease (3.9/10(5)/year; p less than 0.001). Each disease occurred with equal frequency in both sexes and diagnosis of the diseases occurred at similar ages (Spearman's correlation coefficient = 0.84; p less than 0.01). Where ulcerative colitis was common in the various municipalities so was Crohn's disease (Spearman's correlation coefficient = 0.88; p less than 0.001). No cases of Crohn's disease were identified amongst migrants, but 6 cases of ulcerative colitis (prevalence = 85/10(5); 95% confidence interval = 35-195) were found. This difference was significance (p less than 0.05). It seems likely that the two diseases may simply represent the opposite ends of a continuous spectrum of inflammatory bowel disease.