In order to establish criteria for the differential diagnosis between diverticulitis and carcinoma of the sigmoid colon, a study was made comparing 100 patients with proved carcinoma of the sigmoid and 100 patients with every reasonable finding supporting the diagnosis of diverticulitis. The two conditions had many signs and symptoms in common, but certain generalizations were possible. Change of bowel habits, evidence of intestinal bleeding, anemia, and weight loss were more frequent in patients with tumors; pain, fever, and leukocytosis were more frequent in patients with diverticulitis. Roentgenographic study of the sigmoid colon gave the presumptive diagnosis of tumor in 84 patients and of diverticulitis in 29. Sigmoidoscopic examination and biopsy specimens were used as a basis for establishing the diagnosis of cancer in 41 patients.