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Concerns remain over the risk of cancer following differentiated thyroid carcinoma and its causes. Iodine-131 ((131)I) and external irradiation are known to have potential carcinogenic effects. Thyroid carcinoma is a polygenic disease which may be associated with other malignancies. We investigated the incidence of second cancer and its aetiology in a cohort of 875 patients (146 men, 729 women) with differentiated thyroid carcinoma originating from Basse-Normandie, France. Cancer incidence was compared with that of the general population of the Département du Calvados matched for age, gender and period. The cumulative proportion of second cancer was estimated using the life-table method. Factors that correlated with the risk of second cancer were studied using the Cox model. After a median follow-up of 8 years, 58 second cancers had been observed. Compared with general population incidence rates, there was an overall increased risk of second cancer in women [standardised incidence ratio (SIR)=1.52; P<0.01], but not in men (SIR=1.27; P>0.20). Increased risk related to cancers of the genitourinary tract (SIR=3.31; P<0.001), and particularly to cancer of the kidney (SIR=7.02; P<0.01). Multivariate analysis showed that age above 40 years (P<0.01) and a history of previous primary cancer (P<0.001) correlated with risk. In contrast, neither cervical irradiation nor cumulative activity of (131)I was related to the risk. These data confirm that women with differentiated thyroid carcinoma are at risk of developing a second cancer of the genitourinary tract and kidney. Only age and medical history of primary cancer before thyroid carcinoma are risk factors for second cancer. Common environmental or genetic factors as well as long-term carcinogenic effects of primary cancer therapy should be considered.