An abnormal filterability of whole blood through micropore membranes in vitro has been reported in peripheral vascular disease and has been thought to indicate abnormal red cell deformability. Blood from 68 patients with symptomatic peripheral vascular disease of varying severity and from 32 age-matched controls without a history of peripheral vascular disease was studied by the technique of whole blood filtration. In agreement with earlier findings, whole blood filterability was significantly reduced in patients with symptoms of vascular disease, but also their total leucocyte count was significantly higher than that of the controls. Variation in leucocyte count was found to affect significantly whole blood filterability, and the abnormal filterability in peripheral vascular disease could be entirely ascribed to this factor and not to an alteration in red cell deformability. The raised leucocyte count was not due to smoking but its cause could not be explained.