ACUTE mesenteric lymphadenitis is a symptom complex characterized by abdominal pain and tenderness. For the patient suspected of having this condition, the presence of acute appendicitis cannot be excluded either by the history, the physical examination or the laboratory findings. Because of this difficulty in differentiation, with rare exceptions operation is urgently indicated when either diagnosis is made. During the five years between July 1, 1941, and Aug. 1, 1946, there were admitted to Duke Hospital 1,604 patients with the symptoms and signs of an acute condition in the abdomen which led to treatment by appendectomy. Of these, 931 had acute appendicitis. The remaining 673 patients had grossly normal appendixes; in this group, no cause for the abdominal pain was found in 185 patients, while 220 patients had one of a large number of diseases, the most common being hemorrhage from a ruptured graafian follicle, pelvic inflammatory disease and gastroenteritis.