We assessed the relationship between oxidative stress, cytokinetic parameters, and tumor growth in response to novel phospho-nonsteroidal anti-inflammatory drugs (NSAIDs), agents with significant anticancer effects in preclinical models. Compared with controls, in SW480 colon and MCF-7 breast cancer cells, phospho-sulindac, phospho-aspirin, phospho-flurbiprofen, and phospho-ibuprofen (P-I) increased the levels of reactive oxygen and nitrogen species (RONS) and decreased GSH levels and thioredoxin reductase activity, whereas the conventional chemotherapeutic drugs (CCDs), 5-fluorouracil (5-FU), irinotecan, oxaliplatin, chlorambucil, paclitaxel, and vincristine, did not. In both cell lines, phospho-NSAIDs induced apoptosis and inhibited cell proliferation much more potently than CCDs. We then treated nude mice bearing SW480 xenografts with P-I or 5-FU that had an opposite effect on RONS in vitro. Compared with controls, P-I markedly suppressed xenograft growth, induced apoptosis in the xenografts (8.9 ± 2.7 versus 19.5 ± 3.0), inhibited cell proliferation (52.6 ± 5.58 versus 25.8 ± 7.71), and increased urinary F2-isoprostane levels (10.7 ± 3.3 versus 17.9 ± 2.2 ng/mg creatinine, a marker of oxidative stress); all differences were statistically significant. 5-FU's effects on tumor growth, apoptosis, proliferation, and F2-isoprostane were not statistically significant. F2-isoprostane levels correlated with the induction of apoptosis and the inhibition of cell growth. P-I induced oxidative stress only in the tumors, and its apoptotic effect was restricted to xenografts. Our data show that phospho-NSAIDs act against cancer through a mechanism distinct from that of various CCDs, underscore the critical role of oxidative stress in their effect, and indicate that pathways leading to oxidative stress may be useful targets for anticancer strategies.