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While there is very good epidemiological evidence for induction of thyroid cancer by radiation exposure in children, the risk for adults after exposure is still uncertain, especially when concerning relatively small radiation doses. A cohort of 27,415 persons which in 1950 through 1964 had received x-ray treatment for various benign disorders in the locomotor system (such as painful arthrosis and spondylosis) was selected from three hospitals in Northern Sweden. A proportion of this cohort, consisting of 8 ,44 persons (4,075 men and 4,069 women), had received treatment to the cervical spine and thereby received an estimated average dose in the thyroid gland of about 1 Gy. Standard incidence rates (SIR) were calculated by using the Swedish Cancer Register. In the cervical spine cohort, 22 thyroid cancers were found versus 13.77 expected (SIR 1.60, CI 1.00-2.42). The corresponding figures for women were 16 observed cases versus 9.60 expected cases (SIR 1.67: Cl 0.75-2.71). Most thyroid cancers (15 out of 22) were diagnosed > 15 years after the exposure. In the remaining part of the total cohort, i.e. those without cervical spine exposure, no increased risk of thyroid cancer was found (SIR 0.98, CI 0.64-1.38). The study strongly suggests that external radiation exposure of adults at relatively small doses increases the risk of thyroid cancer but also that this increase is very much lower than that reported after exposure in children.