Twenty-five cases of solitary pulmonary chondromatous hamartomas (PCH) were examined by immunohistochemistry to evaluate the mesenchymal and epithelial components. PCH composed of predominantly mature cartilage were designated as C type, those predominantly composed of fibromyxoid tissue as FM type, and those predominantly composed of adipose tissue as A type. FM type PCH revealed various amounts of cartilage in various developmental stages, adipose tissue and fibromyxoid tissue, compared with a uniform pattern of cartilage tissue in C type. The cells of transitional form between spindle cells, stellate cells and chondrocytes were present in FM type. Epithelial components in PCH were bronchial, bronchiolar and cuboidal cells, mostly at the periphery of PCH. S-100 protein consistently stained chondrocytes, stellate and spindle cells in the fibromyxoid tissue of solitary PCH. Fibroblast growth factor was immunolocalized to chondrocytes, spindle and stellate cells in the fibromyxoid tissue. The collagen type was associated with differentiation from primitive mesenchymal cells to chondrocytes (i. e. type I and III collagen appeared in fibromyxoid matrix and type II collagen in the cartilaginous matrix). Fibronectin coordinately appeared with type I and III collagens. The proliferating cell nuclear antigen labeling index of epithelial cells was comparable to those of neoplastic mesenchymal cells, but it was not significantly different between C type and FM type PCH. The primitive mesenchymal cells in the bronchial walls of the control premature neonates were also observed. This immunohistochemical study showed that the progenitor mesenchymal cells in the bronchial and bronchiolar walls may differentiate along chondrocytes, lipocytes, and smooth muscle cells in PCH and that epithelial proliferation is reactive and closely associated with neoplastic proliferation of the mesenchymal component.