Mesenchymal hamartoma of the chest wall (MHCW) occurs as intraosseous expansile mass involving the ribs typically present at birth or in early infancy[1]. The incidence is about 0.03% among primary bone tumors with male predominance, and appropriately 100 cases have been reported. We present a case of 3-month-old female baby with slowly growing left sided chest wall swelling since birth. Computerized tomography (CT) revealed a Approx. 58x55x64mm3 sized expansile heterogeneous soft tissue density lesion (calcific, fluid and soft tissue) arising from posterior aspect of left fourth and fifth ribs with large intrathoracic component resulting in compression of left parenchyma. The lesion causes mass effect in form of widening of 4th and 5th intercostal spaces and shift of mediastinum towards left side. Rest of both lung fields appear normal. Thoracotomy with en bloc excision of the tumor was performed and tissue was sent for histopathological examination. Microscopy revealed structure of well circumscribed lesion chiefly consisting of fascicles of spindled shaped fibroblast admixed with cartilaginous and bony tissue with evidence of endochondral ossification. A final diagnosis of mesenchymal hamartoma of the chest wall was made. Oncologist deferred the chemotherapy The 80% of cases occur prenatally or within first 6 months of life and are usually solitary, but bilaterality or multicentricity also rarely occur[3,4]. Mesenchymal hamartoma is not a true neoplasm, but rather they are hamartomas with focal overgrowth of normal skeletal elements with no propensity for invasion or metastasis. Malignant transformation in MHCW is very rare[6]. The definitive diagnosis is established only by histopathological examination[5]. We hereby conclude that these tumors are very rare and they look aggressive both radiologically and microscopically, so it is essential to be aware of this condition, so that unnecessary over diagnosis and aggressive treatment with chemotherapy and radiotherapy can be avoided considering the fast recovery of the patient when managed with surgery alone.