In serial sections of 75 rectal and colonic biopsies of 42 patients with Crohn's disease the number and size of epithelioid cell granulomas as well as the size of the biopsies were measured. From these data the number of serial sections necessary to detect a possibly existing granuloma was calculated. This number depends on the average size of the sections. A statistical probability of 100% to detect one granuloma is reached by 900 running sections for an average size of 2 mm2 and by 35 running sections for an average size of the sections of 50 mm2.