Hypermethioninemia and absolute methionine intolerance were observed in three siblings. These patients had several peculiar clinical features comprising failure to thrive, mental and motor retardation, facial dysmorphy with abnormal hair and teeth, and myocardiopathy. Hepatic S-adenosylhomocysteine hydrolase activity was decreased by 80% in the three children. These clinical and biochemical features differ from those of hypermethioninemias previously described, and thus represent a new form of inherited disorder of methionine metabolism. Whether S-adenosylhomocysteine hydrolase deficiency is primary or secondary to an unknown metabolic defect remains to be determined.