Reactive oxygen species (ROS), the principal mediators of oxidative stress, induce responses such as apoptosis or permanent growth arrest/senescence in normal cells. Moreover, p53 activation itself contributes to ROS accumulation. Here we show that treatment of p53-null cancer cells with sublethal concentrations of ROS triggered an arrest with some morphological similarities to cellular senescence. Different from a classical senescent arrest in G(1), the ROS-induced arrest was predominantly in the G(2) phase of the cell cycle, and its establishment depended at least in part on an intact Chk1-dependent checkpoint. Chk1 remained phosphorylated only during the repair of double strand DNA breaks, after which Chk1 was inactivated, the G(2) arrest was suppressed, and some cells recovered their ability to proliferate. Inhibition of Chk1 by an RNAi approach resulted in an increase in cell death in p53-null cells, showing that the Chk1-dependent G(2) checkpoint protected cells that lacked a functional p53 pathway from oxidative stress. It has been proposed that the induction of a senescent-like phenotype by antineoplastic agents can contribute therapeutic efficacy. Our results indicate that oxidative stress-induced growth arrest of p53-null tumor cells cannot be equated with effective therapy owing to its reversibility and supports the concept that targeting Chk1 may enhance the effects of DNA-damaging agents on cancer progression in such tumors.