A 15-year-old boy developed thrombocytopenia and purpura two weeks after starting phenytoin therapy. The blood phenytoin level was in the toxic range. There was an increase in immature neutrophils but no abnormalities were present in other cell lines. Recovery was complete after drug therapy was discontinued. Thrombocytopenia is a rare isolated complication of phenytoin therapy. The probable autoimmune etiology distinguishes this syndrome from other phenytoin-induced blood dyscrasias.