The pathogenic mechanisms responsible for inflammatory bowel disease, especially ulcerative colitis (UC), are poorly understood. In animal models, the oral administration of dextran sulfate sodium (DSS) induces colitis, which exhibits several clinical and histological features similar to UC. In addition, the longstanding administration of DSS also induces colon cancer. However, the pathogenic factors responsible for DSS-induced colitis and the subsequent colon cancer also remain unclear. In particular, there are only limited data concerning colonic epithelia cell apoptosis and proliferation in DSS-induced colitis. Therefore, we investigated the relationships between these factors. Colitis was induced in BALB/cA Jcl mice by 8 days of oral administration of standard diets containing 5% (w/w of diet) DSS. The control mice received the standard diets only. Morphological changes in the colonic mucosa were evaluated and scored by light microscopy. Apoptosis was studied by the TUNEL assay, and cell proliferation by Ki-67 immunoreactivity. The macroscopic findings showed the most severe inflammation in the distal colon. Epithelial apoptosis increased approximately 5-fold after DSS administration as compared to the controls. On the other hand, the mitotic cells decreased about half-fold as compared to the controls. Ki-67 immunohistochemistry showed that cells with cell cycle arrest at the G0 stage in the crypt increased approximately 2-fold as compared to the controls. In conclusion, the increased apoptosis and decreased proliferation might lead to a breakdown of the epithelial barrier function, and thus facilitate the mucosal invasion of intraluminal microorganisms in DSS-induced colitis.