Butylated hydroxyanisole (BHA) was given to F344 rats, Syrian golden hamsters and B6C3F1 mice at 2 doses for up to 104 weeks. The two doses were 2.0% and 1.0% for rats and hamsters, and 1.0% and 0.5% for mice. Animals were sacrificed sequentially at 8-week intervals from week 8 to week 104, and the carcinogenic effects of BHA on the forestomach were examined histopathologically. Papillomas and carcinomas were found in rats, hamsters and mice. In rats, papillomas first appeared in week 8 in the group given the higher level of BHA and in week 56 in that given the lower level. The first carcinoma was observed in week 48 in rats given the high level, while no carcinoma was observed in rats given the lower level. In hamsters, papillomas appeared in week 8 in both BHA-treated groups, and in both groups, the incidence of papillomas was much higher than in BHA-treated rats. Squamous cell carcinomas were observed in 4 hamsters (10.0%) among those that survived more than 64 weeks on treatment with the higher level of BHA and in 4 (7.3%) among those treated with the lower level. In mice, papillomas were induced by BHA in both BHA-treated groups after more than 88 weeks. Although the incidence was not statistically significant, carcinoma was also seen in mice, suggesting that BHA may also be carcinogenic to mouse forestomach.