We examined the implication of colitis on the colorectal carcinogenesis in rats. We used 1, 2-dimethylhydrazine (DMH) as a carcinogen and trinitrobenzenesulfonic acid (TNB) as a colitis-inducing agent on F344 rats. After treating the rats with DMH, TNB markedly enhanced the incidence of aberrant crypt foci (ACF), putative preneoplastic lesions, as well as colon cancers in the rats (p<0.01). There was positive correlation between the incidence of ACF and the incidence of tumors. Furthermore, we treated the rats with two different anti-inflammatory drugs (a non-steroidal anti-inflammatory drug:Fenbufen and a platelet activating factor-receptor antagonist:PAF-RA) after pre-treatment with DMH and TNB. Only PAF-RA significantly decreased the incidence of ACF in the rats (p<0.05).