Eight patients with paroxysmal choreoathetosis received phenytoin therapy, starting from 50 mg of oral phenytoin with a gradual increment of the dose. After successful control of the paroxysmal attacks, blood levels of phenytoin in these patients were measured by enzyme immunoassay. The mean phenytoin blood level was 5.2 +/- 3.2 micrograms/ml, with a range of 1.1-10.9 micrograms/ml. It is proposed that phenytoin can be maintained at a lower level in the treatment of paroxysmal choreoathetosis than in seizure control.