A formulation of pentoxifylline (Trental 400) has been claimed to increase claudication distance by improving red cell deformability and decreasing blood viscosity and platelet aggregation. In order to test this claim 30 stable claudicants recruited from one population took part in a double-blind randomised trial of placebo versus pentoxifylline. Both placebo and pentoxifylline treated patients improved subjectively and increased their claudication distance over the study period. Analysis of the blood results showed that the only apparent change was a tendency towards increased red cell filtration in the group treated with pentoxifylline; but no significant difference was found between claudicants and normal controls in the degree of red cell filterability nor did red cell filterability correlate with claudication distance. Pentoxifylline did not affect claudication distance or have any useful effect on blood flow properties, calling into question both its efficacy and suggested mode of action.