OBJECTIVE No randomised trials have addressed the use of external beam radiotherapy (EBRT) in the treatment of differentiated thyroid cancer. The indications for EBRT, the technique and recommended dose all remain controversial. METHODS We included patients treated with EBRT with curative intent from two cancer centres between 1988 and 2001. Data were collected from hospital notes, radiotherapy prescriptions and local cancer registry. RESULTS The indications for treatment in the 41 identified patients were macroscopic residual disease 23 (56%), microscopic residual disease 10 (25%), Hurthle cell variants 3 (7%), multiple lymph-node involvement 3 (7%) and other 2 (5%). Delivered doses ranged from 37.5-66 Gy over 3-6.5 weeks. Rate of local recurrence and overall survival at 5 years were as follows: papillary 26% and 67%; follicular 43% and 48%; well differentiated 21% and 67%; focus of poor differentiation/Hurthle cell variants 69% and 32%; complete excision 25% and 61%; residual disease 37% and 59%; EBRT total dose < 50 Gy 63% and 42%; 50-54 Gy 15% and 72%; > 54 Gy 18%, and 68%. CONCLUSIONS The results in this study are consistent with previous retrospective studies of EBRT. The wide range of indications and doses used with radical intent highlights the lack of clinical and radiobiological data on the response of differentiated thyroid cancer to EBRT. Despite the small study size, the 5-year local recurrence results indicate a possible dose response within the dose range used.