A 33-month-old boy with an inborn error of vitamin B12 metabolism characterized by methylmalonic aciduria, homocystinuria, cystathionuria, and hypomethioninemia had poor vision and a progressive retinal pigmentary degeneration. The child had early growth retardation with microcephaly, developmental delay, and a megaloblastic anemia. The retinal lesions were first noted when he was 1 year of age and, by ophthalmoscopy and by electroretinographic testing, have progressed. Treatment with hydroxocobalamin and L-methionine improved the anemia and the biochemical abnormalities but apparently did not halt the retinal degeneration. We believe the retinopathy is a feature of this disease, particularly in patients with infantile involvement. The retinal lesion may be caused by an unidentified abnormality of sulfur amino acid metabolism.