Four patients are described with persistent atrial fibrillation associated with normal plasma total thyroxine (T4) and triiodothyronine (T3) but an absent plasma thyrotrophin (TSH) response to intravenous thyrotrophin releasing hormone (TRH). Initial cardioversion failed to establish sinus rhythm in three of the four patients. Following specific antithyroid therapy to lower thyroid hormone levels sufficient to allow a normal TSH response to TRH sinus rhythm was established in all four patients, one spontaneously and three after cardioversion. Stable sinus rhythm has persisted in three patients over a 2-yr follow-up period. In the presence of atrial fibrillation, an absent plasma TSH response to TRH should be considered sufficient grounds for antithyroid therapy even if plasma total T4 and T3 are within the expected normal range.