Spontaneous free perforation is an uncommon event in the natural history of Crohn's disease. It occurred in 21 of 1415 patients (1.5%) admitted with Crohn's disease to The Mount Sinai Hospital between 1960 and 1983. The mean duration from onset of Crohn's disease to occurrence of perforation was 3.3 years. Ten patients had small bowel perforation, ten patients had large bowel perforation, and one patient had simultaneous perforation of both ileum and cecum. The incidence of perforation in disease segments of small bowel was 1.0% (jejunum 6.0%, ileum 0.7%), and in the colon, 1.3%. Besides the 21 patients with spontaneous free perforation, an additional nine patients had spontaneous free rupture of an abscess into the peritoneal cavity. The mean duration from onset of Crohn's disease to rupture of abscess was 8.5 years. All 30 patients had surgery within 24 hours of perforation or rupture. All 21 patients with spontaneous free perforations survived, as did all but one of the nine patients with perforated abscess. The cornerstone of the treatment of ileocolonic lesions perforating into the general peritoneal cavity is proximal diversion with delayed reconstruction of intestinal continuity whenever possible. With perforation of the small bowel, primary reanastomosis is possible in selected patients.