The case histories of four patients who developed choreoathetoid movements during intoxication with phenytoin are presented. Drug intoxication was confirmed in each case by measuring the serum phenytoin concentration. Drug interactions were, in part, responsible for the occurrence of intoxication in three of them. Phenytoin intoxication is not always easy to recognize, particularly when nystagmus is minimal or absent, as in these four patients. The estimation of the serum phenytoin concentration is invaluable in this situation.