The records of a series of 700 patients with inflammatory bowel disease, 498 with Crohn's disease and 202 with ulcerative colitis, have been analyzed to determine the relative incidence and characteristic features of their extra-intestinal manifestations. The group with Crohn's disease included 62 with colitis, 223 with ileocolitis, and 213 with regional enteritis. A consideration of the clinical patterns and an understanding of their pathophysiology suggested a subdivision into two main groups: one "colitis related" and one related to the pathophysiology of the small nonspecific third group. Group A, colitis related, comprises joint, skin, mouth, and eye disease. The complications might be immunologically determined, were closely associated with active inflammation, and often responded to medical or surgical treatment of the underlying bowel disease. They occurred in 36% of the entire series of patients: joints were involved in 23%, skin in 15%, and mouth and eye each in 4%. Pyoderma gangrenosum was observed most often in ulcerative colitis and erythema nodosum most often in granulomatous colitis. The incidence of Group A complications was higher in disease involving the colon (42%) than in disease restricted exclusively to the small bowel (23%). There were interrelationships among the various members of Group A, with multiple manifestations occurring in a third of affected patients. Group B, related to small bowel pathophysiology, includes malabsorption, gallstones, kidney stones, and non-calculous hydronephrosis and hydroureter. Disorders in this group were generally related to the severity of the disease in the small bowel and tended to persist even in the absence of active inflammation. In contrast to Group A, this group occurred most frequently in small bowel disease, and least in colonic disease. Malabsorption was virtually confined to the patients with small bowel disease (10% incidence), while gallstones and renal stones were also both more frequent in Crohn's disease (11% and 9% respectively), the latter usually in association with small bowel resection or ileostomy. Group C, found in a small percentage of patients, consists of nonspecific complications, including osteoporosis (3%), liver disease (5%), peptic ulcer (10%), and amyloidosis (1%).