Summary Thyroid cancer incidence has been increasing in many countries, whereas mortality has been falling due to better survival. Radiation is the best-established risk factor and there has been concern that recent rises in incidence might be related to fallout radiation from atmospheric nuclear weapon tests. We examined thyroid cancer time trends and geographical distribution in England and Wales and possible interpretations of these.During Thyroid cancer incidence is increasing in many countries (Pettersson et al., 1991;Weiss, 1979). Exposure to radiation is the main established risk factor. In the USA, the increase in incidence has been associated with widespread use of X-ray therapy for benign conditions of the head and neck among infants and children (deGroot & Paloyan, 1973; Favus et al., 1976;Pottern et al., 1980). Risk is also raised in survivors of the atom bombs of Hiroshima and Nagasaki (Socolow et al., 1963) and populations accidentally exposed to high levels of fresh fission products from nuclear bomb emplosions (Conard et al., 1970). Since nuclear weapon tests release radioactive iodine into the atmosphere, there has been concern that they may have carcinogenic effects on the thyroid (Campbell & Doll, 1963).The risk of thyroid cancer has also been found increased in patients with a history of goitre and other benign thyroid disorders (Preston-Martin et al., 1987;Ron et al., 1987) (ICD7: 194; (WHO, 1957;1967;1977)