Methionine is supposed to be degraded via two known routes, the transsulfuration and the transamination pathways. In particular, patients with hypermethioninemia, due to a defect in the transsulfuration pathway, may catabolize significant amounts of methionine via the transamination pathway. In this study the relative amount of methionine degraded via the transamination pathway in 17 patients with homozygous homocystinuria, due to cystathionine synthase deficiency, was compared with 23 normal subjects, and with a patient with hypermethioninemia due to a deficiency in methionine adenosyltransferase. The homocystinuric patients and the normal subjects were studied in the fasting state as well as after methionine loading (0.1 g/kg body weight). It is concluded that in cystathionine synthase deficient patients, the transamination pathway is not quantitatively important in methionine degradation despite elevated methionine levels. This is in contrast to the patient with methionine adenosyltransferase deficiency, who catabolizes at least 20% of his dietary methionine via the transamination pathway.