Arsenic trioxide (As 2O3) has been established to be an effective agent for treating acute promyleocytic leukemia. Laboratory data suggest that As2O 3 induces apoptosis of several solid tumor cells including lung cancer cells. Regions of tissue hypoxia often arise in aggressive solid tumors, and hypoxic tumors exhibit augmented invasiveness and metastatic ability in several malignancies. Furthermore, hypoxia may impair the treatment efficiency; therefore, we studied the cytotoxic effect of As2O3 on human lung adenocarcinoma cell lines A549 and A549/R (resistant to vincristine, adriamycin and mitomycin etc.) grown under normoxic and hypoxic (1% oxygen) conditions. At both normoxia and hypoxia, 5, 10 and 15 µM As2O3 induced evident growth inhibition and apoptosis in A549 cells as well ...