The association between treatment with lithium and hypothyroidism is well documented. Reports of hyperthyroidism are rare and it is less well known among patients treated with lithium. It may be overlooked simply because the clinician will be watching for hypothyroidism, the reverse phenomenon. This paper describes the cases of four patients who have been on long term lithium treatment, all of whom developed Graves' disease, or an atypical form of hyperthyroidism. Some suggestions are offered to account for the mechanism underlying this unusual association. Although hyperthyroidism may be rare among patients receiving lithium, astute clinical observation and appropriate laboratory tests are called for to detect the early stages of such thyroid dysfunction and to provide appropriate intervention.