Tumor necrosis factor-related apoptosis inducing ligand (TRAIL) shows promise as a chemotherapeutic agent. However, many human cancer cells are resistant to killing by TRAIL. We have previously demonstrated that reovirus infection increases the susceptibility of human lung (H157) and breast (ZR75-1) cancer cell lines to TRAIL-induced apoptosis. We now show that reovirus also increases the susceptibility of human ovarian cancer cell lines (OVCAR3, PA-1 and SKOV-3) to TRAIL-induced apoptosis. Reovirus-induced increases in susceptibility of OVCAR3 cells to TRAIL require virus uncoating and involve increased activation of caspases 3 and 8. Reovirus infection results in the down-regulation of cFLIP (cellular FLICE inhibitory protein) in OVCAR3 cells. Down-regulation of cFLIP following treatment of OVCAR3 cells with antisense cFLIP oligonucleotides or PI3 kinase inhibition also increases the susceptibility of OVCAR3 cells to TRAIL-induced apoptosis. Finally, over-expression of cFLIP blocks reovirus-induced sensitization of OVCAR3 cells to TRAIL-induced apoptosis. The combination of reovirus and TRAIL thus represents a promising new therapeutic approach for the treatment of ovarian cancer.