Forty women who complained of constipation during the third trimester of pregnancy completed 14-day weighed diet records and bowel function charts over a 4-week period. After 2 weeks of baseline observation the women were randomly allocated into three groups which were asked to take 10 g dietary fibre supplements per day in the form of either a corn-based biscuit (Group A), or as wheat bran (Gp B), or to continue without intervention (Gp C). Mean (+/- s.e.m.) daily dietary fibre intake in the first 2 weeks was similar to that in the general population, at 20.4 +/- 1.2 g, for the whole group, and 21.1 +/- 1.6 g for the 26 women who said they had already increased their dietary fibre intakes in attempts to relieve their symptoms. In the final 2 weeks changes in fibre intakes were: Gp A, mean increase 7.2 +/- 1.0 g per day (P less than 0.001); Gp B, mean increase 9.1 +/- 1.6 g per day (P less than 0.001); Gp C mean decrease 3.50 +/- 1.6 g per day (P less than 0.005). These changes were accompanied by an increase in the number of bowel movements and a change to a softer stool consistency in Gps A and B, with no changes in number of bowel movements or stool consistency in Gp C.