We report two patients whose thyrotoxicosis failed to respond to conventional treatment with carbimazole. The patients remained persistently hyperthyroid, both clinically and biochemically, despite several months of carbimazole therapy at the maximum recommended doses. Prednisolone 20 mg per day was then given in addition to the antithyroid drug and a dramatic response observed in both cases. They were then successfully treated with radioiodine. Prednisolone has not been previously used to treat patients with hyperthyroidism who have not responded to thionamide drugs. Our experience suggests that corticosteroids may be potentially useful for controlling thyrotoxicosis in combination with carbimazole before definite treatment can be instituted.