Dietary intervention studies involving the influence of changes in fat or fibre intake on blood pressure in humans are critically reviewed. In each study we asked whether the changes in blood pressure reported were real or spurious. If real, were they due to dietary intervention or to coincidental changes in lifestyle, such as decreased alcohol intake or improved compliance with medication? If changes were due to diet, was the dietary constituent in question responsible? Deficiencies in experimental design in the studies cited made it impossible to resolve whether dietary polyunsaturated fat or fibre affect blood pressure in humans. The review concludes with an outline of those aspects of experimental procedure which need to be incorporated in the design of future studies.