Previous studies have shown that short-term passive cigarette smoking can increase apoptosis in rat gastric mucosa. However, the mechanism is not yet defined. Chloroform and ethanol extracts were used to investigate whether cigarette smoke could induce apoptosis in a human gastric epithelial cell line (AGS) as well as the roles of bcl-2, caspase-3, and cytochrome c in this process. AGS cell lines were treated with either chloroform extract (CE) or ethanol extract (EE) for 5 hours, and the level of bcl-2, the activity of caspase-3, and the level of cytosolic cytochrome c in these cells were determined. Time course studies on the effects of cigarette smoke extracts (CSEs) on DNA fragmentation and cytochrome c relocalization were also performed. Data showed that only CE induced apoptosis in a dose- and time-dependent manner in AGS cells, along with a decrease of bcl-2 and an increase of caspase-3 activity. Pretreatment with Z-DEVD-FMK (specific inhibitor of caspase-3) dose-dependently blocked the DNA fragmentation induced by the CE. Moreover, CE could time- and dose-dependently increase the level of cytochrome c in the cytoplasm, which might activate caspase-3. In conclusion, CSE triggers apoptosis in AGS cells through the inhibition of bcl-2 and the activation of a mitochondria-related pathway.