Abstract An asymptomatic 17-month-old male infant was incidentally found to have a hepatic mass measuring 12 cm in maximum dimension. The histopathologic appearance of the resected mass was that of a typical mesenchymal hamartoma. Spindle cells that comprised a predominant cellular constituent of the lesion were studied immunohistochemically and ultrastructurally. These cells were immunoreactive for vimentin, α-smooth muscle actin (α-SMA), desmin, tenascin, and α B-crystallin, and displayed fibroblastic or myofibroblastic ultrastructural features. These immunohistochemical and ultrastructural findings of the spindle cells were similar to those of “activated” hepatic stellate cells (Ito cells), and they confirmed and reinforced the hypothesis proposed by von Schweinitz et al. that spindle cells forming the predominant cellular constituent of mesenchymal hamartoma are derived from hepatic stellate cells, although another possibility that the spindle cells are derived from myofibroblasts in the portal tracts cannot be ruled out.