Malondialdehyde (MDA) plasma concentrations (marker of lipid free radical peroxidation) and the unfractionated leucocyte and erythrocyte filterability rates (through 5 micron Nuclepore filters in a positive pressure filtration system) were determined during a partial ischaemia induced by two consecutive treadmill tests in 64 stage II peripheral vascular disease (PVD) patients and in 32 matched controls. Transcutaneous oxygen pressure (tcPO2) was monitored constantly throughout the walking tests and recovery periods. In the PVD patients impairments in the flow properties of leucocytes and erythrocytes were associated with increases in MDA plasma concentrations which correlated significantly with a lengthening of the tcPO2 half recovery times--index of the worsening of ischaemia. Patients could be divided into two sub-groups on the basis of these impairments and their half recovery times, suggesting this experimental model may provide a means to identify those stage II PVD patients most likely to develop critical limb ischaemia.