This study was aimed to explore the effects of hyperbaric oxygenation (HBO) alone or combined with As(2)O(3) on proliferation, apoptosis and expression of HIF-1a, VEGF, caspase-3 mRNA of K562 cells, and the molecular mechanism of As(2)O(3) enhancing the anti-leukemic effect of HBO so as to provide a scientific basis for clinical treatment of chronic myeloid leukemia. The effects of drugs on proliferation of K562 cells was assayed by MTT method, the apoptosis rate of K562 cells was detected by flow cytometry with Annexin V/PI double staining, the expressions of HIF-1a, VEGF, caspase-3 mRNA of K562 cells were determined by real-time quantitative PCR. The results showed that as compared with As(2)O(3) alone, HBO combined with As(2)O(3) could increase inhibitory rate of K562 cell proliferation, and enhance apoptotic effect, obviously down-regulate expressions of HIF-1a and VEGF mRNA, up-regulate expression of caspase-3 mRNA. The effect of HBO combined with As(2)O(3) was higher then effect of As(2)O(3) alone, and their effects were synergistic (P < 0.05). It is concluded that HBO combined with As(2)O(3) can increase the expression of caspase 3 mRNA and decrease the expression of HIF-1a and VEGF mRNA, which may be one of molecular mechanisms underlying their synergistic antileukemia efficacy. Wang, Wu, Huang, Hao, Wei, Li, , , (2012). [Effects of hyperbaric oxygenation combined with As(2)O(3) on proliferation of K562 cells and associated mechanism]. Zhongguo shi yan xue ye xue za zhi, 2012 Aug;20(4):863-6. https://www.ncbi.nlm.nih.gov/pubmed/22931643