Coffee which is one of the most popular beverages is under great attention because its attractive pharmacological effects such as anti-cancer properties, antioxidant and cell protective effects. Coffea arabica metabolites like cafestol and kahweol belonging to class of diterpene are expected to be as biotherapeutic drugs for maintaining human health via preventing serious illnesses. However, the understanding of the effects of these two diterpenes against human leukemia cells is still poor. In this study, we investigated the influences of these two coffee diterpenes on the viability and the all-trans retinoic acid (ATRA)-induced superoxide anion (O₂^-)-generating ability of human leukemia U937 cells. Cafestol and kahweol reduced cell viability at a concentration of 50 μM. In addition, 1 μM ATRA significantly reduced viability with cafestol at 48 hr and kahweol at 24 hr respectively. On the other hand, 20 μM these two coffee diterpenes brought about moderate up-regulation of the ATRA-induced O₂^--generating ability. Quantitative RT-PCR and immunoblotting revealed that these two coffee diterpenes significantly up-regulate the ATRA-induced O₂^--generating ability via enhancing gene expression levels of gp91-phox, which is an essential factor for the O₂^--generating ability of leukocytes. These findings demonstrated that cafestol and kahweol show not only the ATRA-enhanced cytotoxicity but also the promoting effects on the ATRA-induced O₂^--generating ability via up-regulation of gp91-phox gene expression.