10 months female child presented with abdominal distension and failure to thrive. On general examination head size was large with open anterior fontanelle. She had intermittent episodes of cough and cold and was admitted and treated as pneumonia. There was history of admission to hospital at the age of 3-4 months for hydrocephalus and ventriculostomy was done. She also had hepatosplenomegaly. Complete blood count revealed anemia with hemoglobin of 8.4 gm/dl, thrombocytopenia with platelet counts of 96,000/cu mm and raised total leucocyte count of 30,000/cu mm. Peripheral smear showed leucoerythroblastic blood picture. Baby was reported to have borderline low levels of sphingomyelinase enzyme. Bone marrow aspiration and biopsy were done to rule out hematological malignancy. Bone marrow aspirate was hemodiluted with 4 % blasts. Bone marrow biopsy showed microscopic findings suggestive of osteopetrosis. There was widening of bony trabeculae with deposition of mature lamellar bone. This caused obliteration of intertrabecular spaces. Prominent osteoclastic activity was noted within howship’s lacunae with focal para trabecular fibrosis on reticulin stain.  Infantogram was done which showed bone within bone appearance with uniformly dense vertebrae.Genetic studies revealed homozygous mutation in gene encoding osteoclast specific proton pump- TCIRG1 Exon 6 c.1258_1264 del confirming diagnosis of malignant infantile osteopetrosis. Family refused further management. Osteopetrosis is important differential diagnosis in infants presenting with hepatosplenomegaly with leucoerythroblastic blood picture and failure to thrive. Suspicion of this rare condition in correct clinical setting may lead to correct diagnosis with simple investigation like X ray. Bone marrow biopsy may not be always needed however, if done can give important diagnostic information.