Angiosarcoma, a malignant tumor of vascular endothelial cell origin, is a lethal disease for which complete cure is rarely seen. The objective of this study was to determine the efficiency of photodynamic therapy (PDT) as a new treatment for angiosarcoma. PDT (630 nm, 25 J/cm2) using Photofrin for a mouse angiosarcoma cell line of human origin (ISOS-1) showed that the rate of cell death increased with increase in the concentration of a photosensitizer (LD50: approximately 2 microg/ml). Furthermore, PDT (630 nm, 100 J/cm2) with Photofrin (5 mg/kg, i.v.) in mice transplanted with ISOS-1 cells resulted in complete disappearance of the tumor in 40% of mice and marked inhibition of tumor growth in the remaining 60%. Significant increases in TUNEL-positive cells and Ki-67-positive cells ware seen 4 h after PDT, indicating that PDT led to not only cell death but also inhibition of the proliferation of angiosarcoma cells. The results show that PDT is effective for treatment of angiosarcoma.