The Chernobyl accident exposed people located hundreds of kilometres away to fallout, but increases in cancer incidence as a result of the accident seem, at present, to be restricted to one tumour type. These thyroid tumours form the largest number of cancers of one type, caused by a single event on one date, ever recorded. Epidemiological, pathological and molecular studies have provided new insights into the carcinogenic process, as well as lessons for future nuclear accidents.