We measured blood copper-containing proteins and plasma total copper in 15 patients with homocystinuria (14 with cystathionine beta-synthase deficiency and one with abnormal cobalamin metabolism), in 13 heterozygotes for cystathionine beta-synthase deficiency, and in 44 normal subjects. Plasma total copper was increased in patients with cystathionine beta-synthase deficiency compared with age- and sex-matched controls; the ratio was 1.41 +/- 0.14 for females and 1.39 +/- 0.15 for males (means +/- SD). This was due to corresponding increases in caeruloplasmin concentrations, but levels were unrelated to total plasma homocysteine. Erythrocyte superoxide dismutase levels were normal. The heterozygotes had normal plasma copper and caeruloplasmin levels. The increased copper and caeruloplasmin may contribute to the precocious atherogenesis occurring in homocystinuria by decreasing the adhesion of vascular endothelial cells to the intima. It is unlikely that decreased lysyl oxidase activity due to chelation of copper by homocysteine is important for the pathogenesis of the connective tissue defect in homocystinuria.