To determine the prevalence of hypothyroidism amongst most adult survivors of childhood cancer in Britain using the British Childhood Cancer Survivor Study (BCCSS). The BCCSS is a population based cohort of individuals diagnosed with childhood cancer between 1940 and 1991 and who survived at least 5 years from diagnosis (n= 17,981). 10483, 71% of those survivors aged at least 16 years, returned a completed questionnaire, which asked if hypothyroidism had been diagnosed. Of the whole cohort, 7.7% reported hypothyroidism with the highest risk among patients treated for Hodgkin's disease (HD) (19.9%), CNS neoplasms (15.3%), Non-Hodgkin's lymphoma (6.2%) and leukaemia (5.2%). Survivors were more likely to develop hypothyroidism if they had received radiotherapy for HD (p= 0.0001) or a CNS neoplasm (p< 0.00005) but not leukaemia (p= 0.3). In these three patient groups, the frequency of hypothyroidism was similar in men and women. Survivors of irradiated CNS tumours reported a prevalence of hypothyroidism, which was substantially lower if discharged to primary care compared with being on hospital follow-up and which declined substantially with increased follow-up in both primary care (p= 0.004) and hospital follow-up (p= 0.023) settings. Hypothyroidism is a common finding amongst adult survivors of childhood malignancy. The substantial differences in reported hypothyroidism prevalence after irradiated CNS neoplasms suggests substantial under-diagnosis, which increased with increased follow-up, and which increased among those followed-up in primary care compared with hospital settings.