Malignant Infantile Osteopetrosis (MIOP) is a rare genetic disorder due to osteoclast abnormal activity. We report a thirteen month-old male patient, diagnosed as MIOP while investigating the cause of hepatosplenomegaly associated with hydrocephalus. His medical history revealed non consanguineous parents and one brother's death at the same age of unknown etiology (similar symptoms). Systemic examination showed hepatosplenomegaly, growth failure, developmental milestones delay, and rickets features. Ophthalmic exam yielded bilateral optic atrophy. Skeleton radiographs detected generalized dense bone and rickets. Cerebral CT scan revealed hydrocephalus. Histological examination showed hypoplastic bone marrow and extra-medullary hematopoeisis. Diagnosis was confirmed by genetic testing that showed two heterozygote mutations within the TCIRG1 gene. The patient received supportive treatment. He died from an acute respiratory distress. MIOP should be kept in mind as a rare cause of hepatosplenomegaly. Early diagnosis and timely Hematopoietic stem cell transplantation are the only curative approach for an otherwise fatal disease.