Eighty-eight healthy omnivores with normal blood pressure participated in a randomized, controlled, cross-over trial of the effect on blood pressure of increasing dietary fibre intake. Subjects were randomly allocated to a control group eating a low fibre diet throughout, or to one of two experimental groups eating a high fibre diet for one of two 6-week experimental periods. Changes in body weight, other dietary constituents and lifestyle factors were avoided as far as possible. Twenty-four hour diet records showed a substantial increase in dietary fibre when subjects were on the high fibre diet. There was no consistent effect of change in dietary fibre intake on group mean systoloic or diastolic blood pressures. Adjusting blood pressures for changes in other dietary components, plasma lipids, electrolytes, body weight and other lifestyle variables did not alter this result. These findings do not support the hypothesis that the blood pressure lowering effect of a vegetarian diet is solely due to an increase in fibre intake.