Ten patients, 5 with recurrent pure cystic and 5 with recurrent mixed cystic-solid thyroid nodules, were studied. When repeated needle aspirations failed to decrease reaccumulation of fluid, tetracycline was injected directly into the cysts. As a result, all the pure cysts resolved completely, and four of the five mixed cystic-solid nodules diminished in size and fluid did not reaccumulate. Follow-up ranged from 12 to 20 months (mean 15 months). The procedure was generally well tolerated. It is concluded that instillation of tetracycline into both recurrent pure cystic and mixed cystic-solid thyroid lesions is a simple, safe and effective treatment.