Cyclooxygenases (COXs) catalyze the synthesis of prostaglandins (PGs) from arachidonic acid. Overexpression of COX-2 is frequently found in human cancers and is suggested to play an important role in tumorigenesis. Recent studies indicated that COX-2 inhibitors exert potent anti-cancer effects on a number of cancers. Interestingly, some COX-2 inhibitors potently induce apoptosis, while other COX-2 inhibitors primarily induce growth inhibition. Therefore, there is a variability in the effects that different COX-2 inhibitors have on cancer cells. In this study, we demonstrated that induction of apoptosis of high COX-2-expressing A549 lung cancer cells by a specific COX-2 inhibitor NS398 was observed in cells cultured under serum-free condition. However, this drug induced G1 growth arrest rather than apoptosis in A549 cells maintained in 10% serum medium. Conversely, low COX-2-expressing H226 lung cancer cells were resistant to NS398-induced apoptosis under both serum-free and serum-containing conditions. Moreover, our results showed that NS398-induced apoptosis is associated with activation of caspase-3, a cysteine protease that plays a crucial role in the execution phase of apoptosis. These results suggest that the cytotoxic effect of COX-2 inhibitors on cancer cells may be influenced by extracellular environments and the anti-cancer action of these inhibitors in vivo needs careful evaluation. Additionally, a correlation between the level of COX-2 expression and the extent of apoptosis induced by COX-2 inhibitors was found.