The records of 81 patients with colitis whose symptoms began after the age of 50 years were analyzed to determine the importance of ischemia as a cause of colitis in this age group and to evaluate the accuracy of previous diagnoses. Patients were classified by clinical, roentgenological and pathologic criteria. A retrospective diagnosis of ischemic colitis was made in three-fourths of the patients, one-half of whom had original discharge diagnoses of ulcerative, Crohn's or nonspecific colitis. This study supports our belief that ischemia is the most common cause of colitis beginning in patients older than 50 years of age. Moreover, the incorrect diagnosis of idiopathic inflammatory bowel disease in a large proportion of these patients may explain why colitis has been reported to behave differently in the elderly than in the young.