To evaluate the efficacy of wheat bran in preventing constipation, 200 hospitalized patients were randomly allocated to groups receiving either a dietary supplement of 40 g bran daily or no dietary supplement at all. A quarter of the bran group patients refused to take their bran from the very beginning (refusers), one third stopped bran consumption during the study (dropouts), and only 42% of the patients continued on bran until discharge or death (participants). Independent of a previous history of constipation, neither the hospital incidence of constipation nor the average percentage of days on laxatives was significantly different between the bran group and the control group. Only the dropouts were significantly more constipated than the control patients, whereas no such difference could be demonstrated in the refusers or participants. It is concluded that the administration of bran as a prophylactic laxative confers no benefit in patients hospitalized for a relatively short time.