Dermatofibroma (DF) and dermatofibrosarcoma protuberans (DFSP) are benign or intermediate malignant fibrotic dermal tumors. The contribution of transforming growth factor β (TGF-β) in the progression of sclerosis in fibrotic diseases has been implicated. To clarify the involvement of TGF-β signaling in the pathogenesis of DF or DFSP, we investigated the expression of thrombospondin-1 (TSP-1), which mediates TGF-β1 activation, on those fibrotic tumors. In the present study, we examined the expression of TSP-1 in DF and DFSP using immunohistochemical analysis and immunoblotting. In immunohistochemical staining, the expression of TSP-1 was detected weakly on epidermis and epidermal appendages, and hardly in fibroblasts in normal skin sections. The expression levels of TSP-1 were elevated in DFSP cells in comparison to normal dermal sections or DF cells. On the other hand, there was no significant difference in the protein levels in vitro of TSP-1 among normal fibroblasts, DF cells, and DFSP cells. Although the contribution of TGF-β signaling to DF or DFSP is still unknown, the expression patterns of TSP-1 in DF cells and DFSP cells may be helpful in differential diagnosis of these tumors.