Four homocystinuric subjects were treated at home with high doses of pyridoxine. Elimination of biochemical abnormalities was achieved, permanently in three patients and transiently in one. Homocystine disappeared from plasma and urine, elevated methionine levels decreased to normal, and an increased fraction of total sulfur was excreted as sulfate. Clinical benefit from treatment was more difficult to demonstrate, but decreased platelet stickiness and increased pigmentation and cystine content of hair were observed. The possible mechanisms for the apparent efficacy of treatment with pyridoxine are discussed.