• Fifty-four pediatric patients with rectal prolapse (RP) were Identified by review of medical records from 1977 to 1987. Rectal prolapse was attributed to chronic constipation (15 patients), acute diarrheal disease (11 patients), cystic fibrosis (CF) (six patients), and neurologic/anatomic abnormalities (13 patients). in nine patients, no underlying cause was identified. The patients with CF did not differ from the other groups In terms of age at time of onset of prolapse, growth measurements, or number of episodes of prolapse. All patients with CF had a history of abnormalities or presented with signs and symptoms consistent with this diagnosis; none had a history of constipation. Although physicians can be reassured that CF is not a likely diagnosis in patients with RP and acute diarrheal disease or a clear history of constipation, a sweat test is indicated in all such cases as well as in those in which there is no apparent underlying cause. A sweat test is not usually indicated in patients with RP in association with underlying anatomic abnormalities.