Serial sections of 1434 colonic and rectal biopsies of 347 patients with Crohn's disease were performed. The relationship between the incidence of granulomas and the inflammatory alterations of the mucosa and some clinical parameters was studied. The presence of granulomas depends on the severity of inflammatory alterations and not on the site of the biopsy. The number of granulomas per mm3 increases from caecum to rectum. The incidence of granulomas decreases with age, duration of illness and under a conservative therapy. If several biopsies, taken at the same or at different times, are studied, granulomas can be found in 40-50% of these patients.