Epithelial and connective-tissue cells were counted in rectal mucosal biopsies from 215 patients with ulcerative colitis, 98 patients with granulomatous colitis, and 50 controls. The results were analyzed statistically. Significantly decreased mucous goblet cells were found both in sigmoidoscopically abnormal ulcerative colitis and in granulomatous colitis, and they increased during the healing process. More pyknotic and karyorrhectic epithelial cells occurred in active ulcerative colitis than in granulomatous colitis. Inactive ulcerative colitis still manifested histologic evidence of acute and chronic inflammation, while sigmoidoscopically normal granulomatous colitis biopsies after previous gross rectal disease showed significantly increased macrophages in the lamina propria. Cell counts were valuable for differential diagnosis after the sigmoidoscopic appearance became normal. The acute inflammation of ulcerative colitis, as indicated by neutrophils, was decreased most notably following therapy with prednisone or 6-mercaptopurine. Chronic inflammation associated with fewer plasma cells was decreased after salicylazosulfapyridine as well as either of the other two drugs; macrophages, indicators of healing, increased most after 6-mercaptopurine combined with another anti-inflammatory agent.