BACKGROUND Anthraquinone is used to make dyes and paper and as a bird repellant. We studied anthraquinone to determine if it caused cancer in rats or mice. METHODS We fed groups of 50 male and female rats feed containing 469, 938, 1,875, or 3,750 parts per million (ppm) anthraquinone for 2 years. Similar groups of male and female mice received feed containing 833, 2,500, or 7,500 ppm anthraquinone. Groups of 50 male and female rats and mice receiving undosed feed served as the control groups. Tissues from more than 40 sites were examined for every animal. RESULTS In each group, the group receiving the highest dose of anthraquinone weighed less than its control group. Male and female rats given anthraquinone had higher rates of tumors of the kidney and urinary bladder. Liver tumors also were increased in female rats and slightly increased in male rats. In male and female mice given anthraquinone, the rates of liver tumors were greatly increased, and a few of these animals developed thyroid gland tumors. CONCLUSIONS We conclude that anthraquinone caused cancer of the kidney and urinary bladder in male and female rats and of the liver in female rats. The occurrence of some liver tumors in male rats may have been related to anthraquinone exposure. We conclude that anthraquinone caused liver cancer in male and female mice, and thyroid gland tumors in mice may have been related to anthraquinone.