Six cases of dermatofibrosarcoma protuberans were studied ultrastructurally. Biopsy material from all six cases as well as cell cultures derived from four cases were examined. In all cases, the tumor cells in vivo and in vitro were related to fibroblasts. The two cases with histiocytoid features and a small spiral pattern exhibited numerous lipid vacuoles but no histiocytic cell markers. In these two cases, cultured cells contained a higher number of intracytoplasmic vacuoles than the control fibroblasts. Two other cases exhibited some basement-membrane-like material surrounding tumor cells. All of the investigated cell strains obtained from the tumors showed synthesis of collagenous proteins similar to that found in fibroblasts. However, the level of total collagen was reduced, and type-III collagen was absent. The morphological variations in these cases of dermatofibrosarcoma protuberans appeared to be related to the histological pattern and may reflect the heterogeneity of normal fibroblasts.