Mild homocysteinemia occurs surprisingly often in patients with premature vascular disease. We studied the possible enzymatic sources of this mild hyperhomocysteinemia and the control of homocysteine levels in plasma by treatment of patients with the cofactors and cosubstrates of homocysteine catabolism. We assessed homocysteine metabolism in 131 patients who had premature disease in their coronary, peripheral, or cerebrovascular circulation by using a standard oral methionine-load test Impaired homocysteine metabolism occurred in 28 patients. We assayed levels of the primary enzymes of homocysteine catabolism in cultured skin fibroblast extracts from 15 of these 28 patients. The patients' cystathionine 0-synthase levels (3.68±2.52 nmol/h per milligram of cell protein, mean±SD) were markedly depressed compared with those from 31 healthy adult control subjects (7.61 ±4.49,P<.001). The patients' levels of 5 -methyltetrahydrofolate: homocysteine methyltransferase were normal. While betalne: homocysteine methyltransferase was not expressed in skin fibroblasts, 24-hour urinary betaine and fyiV-dimethylglycine measurements were consistent with normal or enhanced remethylation of homocysteine by betaine: homocysteine methyltransferase in the 13 patients tested. When treated daily with choline and betaine, pyridoxine, or folic acid, there was a normalization of the postmethionine plasma homocysteine level in 16 of 19 patients. Our results indicate that mild homocysteinemia in premature vascular disease may be caused by either a folate deficiency or deficiencies in cystathionine β-synthase activity. It does not necessarily involve deficiencies of either 5-methyltetrahydrofolate: homocysteine methyltransferase or betaine: homocysteine methyltransferase. Effective treatment regimens are also defined.