Young bracken fern is used as a human food in Japan and other countries. However, it has been demonstrated that bracken is carcinogenic to cattle and laboratory animals such as rats, mice, and guinea pigs. Rats fed a diet containing bracken fern developed tumors of the ileum and urinary bladder. Mammary cancer was also induced in Sprague-Dawley rats fed a bracken diet. Study of the human cancer risk of this plant is thus of great importance. The fern is usually used as a food-stuff in Japan after it is treated with plain boiling water or boiling water containing wood ash or sodium bicarbonate; sometimes, bracken is pickled in salt. Although the carcinogenic activity of processed bracken thus prepared was reduced markedly, weak activity was still retained. This paper deals with the carcinogenicity of unprocessed and processed bracken fern to laboratory animals and its human cancer risk.