Malignant infantile osteopetrosis (MIOP) is a hereditary bone disorder caused by osteoclastic dysfunction. Within the first year of life affected children present with recurrent infections, vision impairment, failure to thrive, bone marrow failure, and at later stages neurological deficits and ultimately death. Bone marrow transplant (BMT) is the only curative treatment. We present a patient with MIOP, who showed the first symptoms at three weeks of age, and the disease was diagnosed at 11 months of age. The boy had a successful BMT after which the delayed psychomotor development improved.