Background: Disorders of intracellular cobalamin (Cbl) metabolism are classified from A to J according to biochemical phenotype, and genetic and complementation analyses. CblD-deficient patients present with developmental, hematologic, neurologic, and metabolic findings. Clinical Observation: An 11-year-old boy presented with neutropenia, increased mean corpuscular volume, psychomotor retardation, and seizures. His plasma total homocysteine and urinary methylmalonic acid levels were elevated, and a homozygous nonsense mutation [p. R250X (c.748C>T] leading to premature termination of translation was identified in the MMADHC gene, which was compatible with CblD defect. Conclusion: In the presence of increased mean corpuscular volume and other hematologic manifestations, such as leukopenia, thrombocytopenia, and megaloblastic anemia, with severe nonspecific or mild neurologic symptoms, Cbl synthesis defects should be considered.