Objective: To examine the effect of dietary fiber intake on blood pressure (BP). Design: Randomized, double-blind, placebo-controlled trial. Setting and participants: A total of 110 trial participants aged 30 to 65 years who had untreated, but higher than optimal BP or stage-1 hypertension were recruited from the community in New Orleans, Louisiana, USA. Interventions: Study participants were randomly assigned to receive 8 g/day of water-soluble fiber from oat bran or a control intervention. Main outcome measures: Nine BP measurements were obtained by trained observers using random-zero sphygmomanometers, over three clinical visits, at the baseline and termination visits of the trial. An average of the nine measurements was used to determine mean BP at the baseline and termination visits. Results: The net changes [95% confidence interval, (Cl)] in systolic blood pressure were -1.8 mmHg (-4.3 to 0.8, P = 0.17) following 12 weeks, -2.2 mmHg (-5.3 to 1.0, P = 0.18) following 6 weeks, and -2.0 mmHg (-4.4 to 0.3, P = 0.09) for an average of the 6- and 12-week visits. The corresponding net changes (95% Cl) in diastolic blood pressure were -1.2 mmHg (-3.0 to 0.5, P = 0.17) following 12 weeks, -0.8 mmHg (-3.1 to 1.4, P = 0.47) following 6 weeks, and -1.0 mmHg (-2.6 to 0.5, P = 0.19) for an average of the 6- and 12-week visits. Conclusions: Our findings suggest that a diet rich in fiber may have a moderate BP-lowering effect and indicate the need for further investigation of this important question. © 2004 Lippincott Williams & Wilkins.