Background Mesenchymal hamartoma of the chest wall is an extremely uncommon lesion of infants. Radialogic features simulate a malignant neoplasm; however pathologic examination demonstrates an admixture of fibroblasts, benign cartilage and woven bone. There is a paucity of cytologic reports of this rare entity in the available literature. Case A 5-month-old infant presented with a large right chest wall swelling. Radiologic investigation demonstrated lytic destruction of the right 7th, 8th and 9th ribs with a large soft tissue mass. Fine needle aspiration smears showed lobules of hyaline cartilage and a few spindle cells with abundant chondromyxoid matrix. A cytologic diagnosis of a benign chondroid, possibly hamartomatous lesion was given, which was confirmed is mesenchymal hamartoma on histopathologic examination. Conclusion Mesenchymal hamartoma is a rare chest wall lesion that can be diagnosed on cytology, provided that the cytopathologist is aware of this uncommon entity and appreciates the benign cytologic features. (Acta Cytol 2010; 54:63-65)