The tumorigenicity of kojic acid (KA), which is widely used for food and cosmetics in Japan, was examined in B6C3F1 mice. Female and male animals were divided into three groups and given 0, 1.5 and 3.0% KA containing food from the age of 6 weeks. At sacrifice after 20 months, thyroid weights were significantly increased in both sexes of mice receiving KA, especially in the male groups. The enlarged thyroid glands histologically featured diffuse hyperplasia and follicular adenomas, the incidences of the latter being 65% and 87%, respectively in 1.5% and 3.0% KA-treated males, significantly higher than the control value of 2%. In the females, the figures were 2%, 8% and 80% in the 0%, 1.5% and 3.0% KA groups, respectively. The serum free T3 levels in the 3.0% KA animals of both sexes at month 6 were significantly lower than in the controls. On the other hand, their serum TSH levels were higher, although the differences disappeared at later time points. In conclusion, continuous administration of high dose of KA induces thyroid adenomas in male and female B6C3F, mice, presumably by a mechanism involving decrease in serum free T3 levels and increased TSH.