We reviewed the clinical characteristics of 10 patients with thyrotoxic periodic paralysis. In these patients, a relatively uniform group of young men, the periodic paralysis developed nearly concurrently with the onset of hyperthyroidism. The attacks were precipitated most frequently by rest and by exercise and, occasionally, by ingestion of a large carbohydrate load. In each patient, the paralysis resolved on return of euthyroidism. The approximate incidence rate for thyrotoxic periodic paralysis in our largely white North American patient population (all hyperthyroidism cases) ranged from 0.1% to 0.2%, which is one tenth the rate reported for Oriental populations. In 7 patients, electrodiagnostic testing revealed characteristic changes in compound muscle action potential amplitude in response to exercise of the muscle being tested.