Methylmalonic Acidemia (MMA) is a rare inborn error of metabolism which can be associated with pancytopenia and in some cases complicated of severe infections. We report the case of a 1 month old infant presented with severe infectious clinical and biological features including pancytopenia and hypogammaglobulinemia. He was considered and treated as immunodeficient. After his death the diagnosis of methylmalonic acidemia was confirmed by the presence of elevated methylmaIonic acid concentration in the urine by gas chromatography. In fact patients with MMA develop marrow hypoplasia and trilineage dysplasia especially during metabolic crises. Severe compromise of the immune system has been associated with a number of well-defined biochemical abnormalities.