A three-month-old girl was admitted to the hospital with a history of pallor. On physical examination, the liver was enlarged and a solid mass was palpated in the left abdomen. Laboratory evaluation revealed anemia and thrombocytopenia. Bone marrow was hypocellular with reduced number of megakaryocytes. Radiographic findings and scintigraphic study of the long bones were consistant with osteopetrosis. In the imaging studies, including ultrasonography, computerized tomography, magnetic resonance imaging and scintigraphic study, an ectopic spleen expanded into the bony pelvis was observed. We report here unique case of infantile osteopetrosis associated with pelvic ectopic spleen.