The role of cytochrome P450 2J2 (CYP2J2) in cyclophosphamide (Cy) bioactivation was investigated in patients, cells and microsomes. Gene expression analysis showed that CYP2J2 mRNA expression was significantly (P<0.01) higher in 20 patients with hematological malignancies compared with healthy controls. CYP2J2 expression showed significant upregulation (P<0.05) during Cy treatment before stem cell transplantation. Cy bioactivation was significantly correlated to CYP2J2 expression. Studies in HL-60 cells expressing CYP2J2 showed reduced cell viability when incubated with Cy (half maximal inhibitory concentration=3.6 mM). Inhibition of CYP2J2 using telmisartan reduced Cy bioactivation by 50% and improved cell survival. Cy incubated with recombinant CYP2J2 microsomes has resulted in apparent Km and Vmax values of 3.7-6.6 mM and 2.9-10.3 pmol/(min·pmol) CYP, respectively. This is the first study demonstrating that CYP2J2 is equally important to CYP2B6 in Cy metabolism. The heart, intestine and urinary bladder express high levels of CYP2J2; local Cy bioactivation may explain Cy-treatment-related toxicities in these organs.