Hoffman D A (Environmental Epidemiology Branch, National Cancer Institute, Room 3c–07, London Bldg., Bethesda MD 20205, USA), McConahey W M, Fraumeni, J F and Kurland L T. Cancer incidence following treatment of hyperthyroidism. International Journal of Epidemiology, 1982, 11: 218–224 A retrospective cohort study of women treated for hyperthyroidism at the Mayo Clinic was conducted to evaluate the risk of cancer according to type of therapy. One or more years after the start of treatment, there were 105 cases of cancer observed among 1005 women treated with radioiodine (131I) and 247 cases among 2141 women treated with surgery. No difference was observed between the two study groups for total cancer incidence (RR = 1.0), breast cancer (RR = 0.8), or leukaemia (RR = 0.6). Although based on small numbers of cases, an elevated risk of cancer was observed in organs that concentrate 131I (salivary glands, digestive tract, kidney and bladder) (RR = 1.8). While the findings are suggestive, they indicate the need for larger surveys to evaluate the carcinogenic potential of 131I.