A case of a four-month-old infant diagnosed as cartilaginous hamartoma of the rib is presented. This rare tumor usually presents at birth. The patient had respiratory distress syndrome. Swelling of the ribs was palpable on physical examination and the infant underwent surgery for excision of the ribs. Histopathologically, the tumor showed benign focal overgrowth of normal skeletal elements with cartilaginous, vascular and primitive-appearing mesenchymal elements. Additionally, secondary aneurysmal cyst formation coexisted with the tumor. The diagnosis was infantile cartilaginous hamartoma of the rib. In this entity, primitive-appearing mesenchymal stroma may be mistaken for a malignant condition. Usually a benign clinical course is expected and treatment is by block excision.