The effect of unprocessed bran in a dose of 5 g three times daily and a dose of 2.5 g three times daily for one year on the recurrence rate of anal fissures was studied in a double-blind, placebo-controlled trial in 90 patients with recently healed acute posterior anal fissures. Fifteen patients (16.6%) were withdrawn before the code was broken due to failure to follow the trial protocol for various reasons. Significantly fewer recurrences occurred in patients receiving bran 5 g three times daily (recurrence rate 16%, 95% confidence limits, 4.54 to 36.08) when compared with patients receiving bran 2.5 g three times daily (60%; 38.67 to 78.87) (P less than 0.01) and with patients receiving placebo three times daily (68%; 46.50 to 85.05) (P less than 0.01). No significant difference in recurrences was found between patients on bran 2.5 g and those on placebo.