We report a pediatric patient with hyperimmunoglobulin E syndrome (HIES) treated with recombinant IFN-gamma (rIFN-gamma) for 2 1/4 years who developed autoimmune thrombocytopenia and was positive for serum antiplatelet antibody and antinuclear antibody (ANA). She was then treated with i.v. methylprednisolone pulse therapy followed by oral immunosuppressive drugs. With this therapy, her platelet count increased and was maintained within the normal range for more than a year. We retrospectively examined her sera stored at -40 degrees C for ANA and found that the ANA level was increased from 1:40 to 1:640 with the rIFN-y therapy. Therefore, we believe that, in this case, rIFN-y treatment may have played a crucial role in triggering autoimmune thrombocytopenia. Furthermore, this case demonstrates that caution must be observed in administering rIFN-gamma to genetically predisposed patients.