The expression of thrombospondin (TSP) of hepatocellular carcinoma (HCC) was investigated by light and electron microscopic immunohistochemical studies, using an anti-human TSP antibody to HCC and liver specimens. Tissue specimens were obtained by surgical resection from 60 patients with HCC. In light microscopic immunohistochemistry, all specimens showed strong immunoreaction for TSP in fibrous tissues in HCC and surrounding nonmalignant areas. In the strong TSP-stained fibrous tissue, immunoreaction for TSP was observed in fibroblasts and vascular endothelial cells. In 31 of 60 cases, HCC cells revealed cytoplasmic staining for TSP. In electron microscopic immunohistochemistry, reaction products for TSP were revealed in the rough endoplasmic reticulum (RER) of HCC cells, and also in the RER of fibroblasts and endothelial cells. These findings proved that TSP was synthesized in HCC cells, in endothelial cells, and in fibroblasts surrounding the tumor, and further suggested that TSP in the fibrous tissues surrounding carcinoma was basically synthesized by nonparenchymal cells such as fibroblasts and endothelial cells.