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Anew case of cobalamin C disease associated with hemolytic-uremic syndrome (HUS) in the neonatal period is described. A 28-day-old boy presented with failure to thrive, hypotonia, pancytopenia, and features of HUS (microangiopathic hemolytic anemia, thrombocytopenia, and renal failure). The possibility of the diagnosis of an underlying vitamin B12 disorder was prompted by evidence of megaloblastic changes on the peripheral smear and by finding in the literature a suggested association of neonatal HUS with this cobalamin-related metabolic disorder. Amino acid analysis showed elevated homocysteine levels in the plasma and increased levels of both homocysteine and methyl malonic acid in the urine. Diagnosis of cobalamin C disease was confirmed by complementation studies using skin fibroblasts. Therapy included parenteral hydroxocobalamin, carnitine, and leucovorin calcium (folinic acid). Cobalamin C disease should be considered in the diagnosis of patients presenting with HUS in infancy who have unexplained megaloblastosis, pancytopenia, neurologic impairment, and failure to thrive. Early diagnosis and institution of therapy may be effective in improving survival and quality of life.