The effects of the synthetic antioxidant butylated hydroxyanisole (BHA) and naturally occurring antioxidants such as L-ascorbic acid (AsA; vitamin C), citric acid, benzoic acid or gallic acid in combination, on the development of rat forestomach epithelial lesions were investigated. A dietary level of 2.0% was selected for all antioxidants and the experimental period was 1 year. As compared with BHA alone, treatment with AsA in combination with BHA increased the severity of the hyperplasia in the mid-region, but not in the prefundic region of the forestomach epithelium, as well as the multiplicity of forestomach tumors. Furthermore, incidences of squamous cell carcinomas (SCCs) in the forestomach were significantly elevated in this group, while papilloma development only showed a tendency to increase. In addition, SCC invasion into the muscle layer of the forestomach was observed only in the BHA + AsA group. Treatment with the other antioxidants did not affect BHA-induced forestomach lesion development. The present study demonstrated that AsA can clearly enhance BHA forestomach carcinogenesis.