A 1.5-year-old girl developed high frequency tremors and chorea after receiving a dose of 120 mg/kg/d trimethoprim-sulfamethoxazole (TMP-SMX) for the treatment of Pneumocystis pneumonia. The child was human immunodeficiency virus-negative but immunocompromised because of prolonged immunosuppressive therapy. These symptoms disappeared 3 days after TMP-SMX was discontinued. Pediatricians should be aware of tremors and chorea among the potential adverse effects of high doses of TMP-SMX.