The fine structures of three fibrous hamartomas of infancy were studied. All three components of these lesions were examined. The principal cells in the fibrous and myxoid areas were fibroblasts. Cells with stellate cytoplasmic projections were more prominent in myxoid areas. Myofibroblasts were abundant in two cases and not in the other. The adipose tissue component was formed by lipocytes and occasional preadipose fibroblasts. Blood vessels were more numerous in myxoid areas. Larger vessels were accompanied by smaller vessels and by clusters of cells with primitive junctions, suggesting early blood vessel formation. Some small vessels were surrounded by concentric layers of mesenchymal cells, as if attempting to form media. Electron microscopy seems to confirm the hamartomatous nature of fibrous hamartomas of infancy. The lesion appears to recapitulate the formation of blood vessels and fat, as seen in fetal tissues. Cellular myxoid areas showed prominent vasoformative proliferation, which decreased in the adipose tissue and was least prominent in the dense fibrous component. The latter may represent the end stage of the fibrous proliferation.