In January 2001 Dioxin was listed as a 'Known Human Carcinogen' in the US Federal Government's Ninth Report on Carcinogens. This means that the National Toxicology Program had found that there was sufficient evidence that dioxin caused cancer from studies in humans and from a knowledge of the causative mechanism.

Reference: National Institute of Environmental Health Sciences, TCDD - Dioxin - is Listed as 'Known Human Carcinogen', Press Release, 19 January 2001.