Cobalamin C deficiency (CblC) is the most frequent inborn error of cobalamin (Cbl) metabolism, which has a wide clinical spectrum. Cbl C defect causes the accumulation of methylmalonic acid and homocysteine and decreased methionine synthesis. Here we presented two distinct clinical forms of patients with CblC. First patient with early onset form was presented with failure to thrive, mild hypotonia, megaloblastic anemia and leukopenia at 2.5 months old. Second patient was presented with mental status changes, loss of speech, inability to walk and megaloblastic anemia at 12 years old. Laboratory analysis showed hyperhomocysteinemia, low plasma methionine levels and high urinary methylmalonic acid in both patients. Molecular analysis supported the diagnosis of CblC and treatment resulted in improvement of biochemical abnormalities, and neurologic findings in both patients.

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