The generation of toxic oxygen metabolites is more usually associated with inflammation. However, pathological free radical reactions can cause tissue damage by adversely affecting prostacyclin (PGI2) synthesis allowing initiation of coagulation. We have assessed changes in the red cell defence to toxic oxygen metabolite generation, viz measurement of glutathione concentration (GSH) and superoxide dismutase activity (SOD). GSH and SOD were measured in 20 patients with peripheral arterial disease, 22 patients with vasculitis, and 11 patients with angina, and compared to 17 matched controls. The 53 subjects with arterial disease had significantly lower SOD levels: in contrast GSH levels were significantly higher. Extracellularly plasma thiol levels (PSH) were low and caeruloplasmin (Cp) levels were high. We suggest that free radical pathology exists not only in inflammatory vascular disease but also in atherosclerosis.