Caffeine overrides checkpoints in the G2 phase of the cell cycle by inhibiting DNA repair at this phase and increases the cytotoxicity of antitumor drugs, such as cis-diamminedichloroplatinum (CDDP). The enhanced cell death induced by caffeine is characterized by apoptosis. In this paper, we demonstrate that this apoptotic event occurs in S phase of the cell cycle, whereas CDDP induces a block in G2/M. DNA histogram analysis revealed that caffeine reduced G2 arrest in CDDP-treated EL-4 cells. In a synchronous population, the ratio of cyclin B:p34cdc2 was upregulated just before the cells went into the apoptotic pathway. A rapid increase in DNA fragmentation was detected at 12-24 h, when marked regression of G2/M phase was observed. Moreover, the degree of DNA fragmentation in CDDP + caffeine-treated cells was not reduced when the cell cycle was arrested at metaphase by exposure to the spindle-inhibitor nocodazole. It is possible that execution of the apoptotic program after treatment with caffeine did not require the EL-4 cells to reenter G1 phase. The apoptotic cell fraction in the group of CDDP + caffeine was recognized as an S population by bivariate analysis of apoptosis and DNA content. These results suggest that enhancement of the apoptotic activity of CDDP-treated cells by caffeine is not a G1-phase event but an S-phase-specific event, whereas cells were arrested in G2/M phase, and that it is regulated by G2 checkpoint-related proteins.