Serial sections of 1047 colonic and rectal biopsies from 385 children and adolescents suffering from Crohn's disease were studied histologically. Inflammatory alterations were seen in 76% of the biopsies and 84% of the patients. Incidence of inflammatory infiltration, of crypt abscesses, of erosions, and of ulcers decrease from caecum to rectum were studied. The incidence of granulomas falls along the colon but rises in the rectum. The occurrence of granuloma depends rather on the severity of the inflammation than on the biopsy site. Granulomas are present in 26% of the biopsies and 42% of the patients. Compared to adults, the incidence was twofold in children. Discontinuous type of infiltration, density of infiltration, crypt abscesses, rectal erosions prevail in children, diffuse type of infiltration, and colonic ulcers in adults. Incidence of granulomas is reduced after the second year of illness and after the 16th year of life. The number of granulomas per mm3 biopsy is increased in children, their average size in adults.