Photodynamic therapy is a promising treatment modality for a variety of cutaneous neoplasms and other skin disorders. Studies suggest an involvement of multiple pathways during photodynamic-therapy-mediated cell death. A complete knowledge of the mechanisms involved in photodynamic therapy may lead to an improvement in its therapeutic efficacy. In vitro as well as in vivo studies have shown the involvement of apoptosis during photodynamic- therapy-mediated cell death. The pathways by which photodynamic therapy causes this are not fully understood. In this study, employing human epidermoid carcinoma (A431) cells and silicon phthalocyanine 4 photodynamic therapy, we show that the cell surface death receptor Fas (also known as APO-1 or CD-95) pathway is an important contributor to photodynamic-therapy-mediated apoptosis. Employ- ing flow cytometric analysis and confocal microscopy we first established that silicon phthalocyanine 4 photodynamic therapy results in a significant induction of apoptosis in A431 cells. Immunoblot analysis revealed a significant time-dependent increase in the protein expression of Fas at 5, 15, 30, and 60 min post-photodynamic therapy followed by a decrease at later time-points (2 and 3 h post-photodynamic therapy). A Fas enzyme-linked immunosorbent assay demonstrated an increase in this protein in cell culture medium starting at 1 h post-photodynamic therapy and showing a time-dependent response up to 3 h following therapy, suggesting a diffusion of soluble Fas from cells into the medium from 1 h after photodynamic therapy. Silicon phthalocyanine 4 photodynamic therapy also resulted in a time-dependent increase in (i) the multimerization of Fas protein, (ii) the protein expression of Fas ligand, (iii) FADD, an adapter molecule for Fas, and (iv) the binding of FADD with Fas. Silicon phthalocyanine 4 photodynamic therapy also caused a significant activation of FLICE, as evident from the appearance of cleaved products of pro-caspase 8. Further, a pretreatment of cells with rhFas:Fc fusion protein or general caspase inhibitor Z-VAD-FMK followed by silicon phthalocyanine 4 photodynamic therapy resulted in a significantly enhanced cell survival. Taken together, our data, for the first time, delineate an involvement of the Fas pathway as an important contributor to photodynamic-therapy-mediated apoptosis of cancer cells. These observations may be important for improving the efficacy of photodynamic therapy for the treatment of skin cancer and possibly other skin disorders. J Invest Dermatol 115:1041-1046 2000