Near-infrared spectroscopy has been performed on the calf muscles of 38 subjects, 21 normal controls without vascular disease and 17 patients with peripheral vascular disease. Oxygen consumption was measured in the calf by calculating the rate of conversion of oxyhaemoglobin to deoxyhaemoglobin during a period of tourniquet-induced ischaemia. Postischaemic reoxygenation was also measured. Median oxygen consumption in patients with peripheral vascular disease was 0.10 ml 100 g tissue-1 min-1, while in the control group it was 0.20 ml 100 g tissue-1 min-1 (P less than 0.03, Mann-Whitney U test). The median time taken to reach maximum oxyhaemoglobin levels after ischaemia was 40 s in patients with peripheral vascular disease and 20 s in controls (P less than 0.02). The results indicate that oxygen consumption is reduced in peripheral vascular disease. Near infrared spectroscopy is a non-invasive method for assessing metabolic improvement resulting from surgical or pharmacological treatment.