Surgeons operating on patients with an obscure peritonitis should be aware of the diverse etiologies of small intestinal perforation and the general principles of management of each. A series of 16 adult patients with free perforation of the small intestine and spreading peritonitis in the absence of bowel obstruction, incarcerated hernia, or trauma is reviewed. Etiologies were as follows: Crohn's disease, four patients; foreign body ingestion, two patients; jejunal diverticulosis, one patient; lymphoma, two patients; cancer chemotherapy, one patient, amyloidosis, one patient; idiopathic, five patients. Although all patient presented with diffuse peritonitis, the findings of fever and leukocytosis were inconstant. Free air was demonstrated on radiographs in only eight of 16 patients, and the correct preoperative diagnosis was not made except in the four patients with Crohn's disease. Resection and primary anastomosis were utilized successfully in ten patients, the remainder of the patients undergoing oversewing the the perforation. Four patients (25%) died.