Using clustering analysis, we sought to identify groups of patients on the basis of the disease course among a population of 177 patients with Crohn's disease and followed for 3 years or more, starting from the first frank exacerbation of the disease. The first 36 values of a monthly clinical score represented the active variables of the clustering analysis. This method yielded 2 course groups, A and B, of 95 and 82 patients respectively. The unfavorable course in group A was characterized by the persistence of the clinically active disease at 3 years, whereas group B individuals achieved complete clinical remission within 2 years of onset on the average. Among the initially known clinical data which could explain the course, only the incidence of an occlusive syndrome was higher in group B, which showed a more favorable course. Although we applied clustering analysis to a patient sample over a period of only 3 years, our results do suggest the existence or 2 primary course groups within the population of patients with Crohn's disease. It would appear that the disease course cannot be predicted from the clinical parameters present at the time of onset, but rather becomes apparent during the course of the first 2 years.