Five human soft tissue sarcoma (STS) cell lines (HTB-82 rhabdomyosarcoma, HTB-91 fibrosarcoma, HTB-92 liposarcoma, HTB-93 synovial sarcoma and HTB-94 chondrosarcoma) were analysed for their sensitivity to tumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) and the function of the TRAIL apoptotic pathway in these cells. TRAIL induced significant apoptosis (>90%) in HTB-92 and HTB-93 cells, whereas no effect was observed in HTB-82, HTB-91 and HTB-94 cells. TRAIL-Receptor 1 (TRAIL-R1) was expressed in TRAIL-sensitive HTB-92 and HTB-93 cell lines, but not in TRAIL-resistant HTB-91 and HTB-94 cells. HTB-82 cells, which expressed the long (c-FLIP(L)) and short (c-FLIP(S)) splice variants of the FLICE-like inhibitory protein (FLIP), were resistant to TRAIL in spite of the presence of TRAIL-R1. TRAIL-R2,-R3,-R4 and osteoprotegerin (OPG) expression did not correlate with TRAIL sensitivity. Coincubation of TRAIL and doxorubicin led to the overexpression of TRAIL-R2 resulting in a synergistic effect of doxorubicin and TRAIL in TRAIL-sensitive cell lines and in the overcoming of TRAIL-resistance in all of the TRAIL-resistant cell lines, except HTB-91, which lacked caspase 8 expression. These data suggest that TRAIL, either as a single agent or in combination with cytotoxic agents, might represent a new treatment option for advanced STS, which constitutes a largely chemotherapy-resistant disease.