Objective To investigate the effect of silencing heme oxygenase-1 ( HO-1) on the progression of leukemia in xenografted mouse models of acute monocytic leukemia (M5). Methods U937 cells were infected with a lentivirus to silence expression of HO-1. Then, modified U937 cells were subcutaneously injected into the armpits of NOD/ SCID mice to establish M5 xenografted mouse models that were divided into blank and experimental groups ( U937, vehicle, and siHO-1 groups). Subsequently, the tumor formation time, tumor volume, tumor infiltration into surrounding tissues, peripheral white blood cell and platelet counts, hemoglobin content, and survival time of the mice were assessed. Results Fluorescence microscopy and Western blot confirmed successful infection. After formation of an axillary mass, an increased white blood cell count and leukemic cells in peripheral blood and expression of CD13, CD14, and CD64 in bone marrow indicated successful establishment of the xenografted mouse models. Compared with the U937 and vehicle groups, there was a significant increase in disease latency in the siHO-1 group (P<0.05). Xenografted mouse models with HO-1 downregulation developed tumors more slowly (P< 0.05), and the peripheral white blood cell count was increased and platelets were decreased more slowly(P<0.05). Mice in the HO-1 downregulated group survived significantly longer than those in the other two groups (P<0.05). Conclusions Silencing expression of HO-1 may slow the progression of leukemia in M5 xenograft mouse models.