We report the first known case of daunorubicin administered directly into the human central nervous system. A 3 1/2-year-old female with pneumonia and otitis media was diagnosed with acute lymphoblastic leukemia and was admitted for antibiotics and chemotherapy. On the first day she inadvertently received a 17 mg intrathecal (IT) injection of daunorubicin. When the error was recognized about 1 hour later, her cerebrospinal fluid (CSF) was exchanged with sterile saline by barbotage, IT hydrocortisone was given, a subarachnoid catheter was inserted, and the CSF was allowed to drain for 36 hours. Only 5.6 mg (33%) of the dose was recovered from CSF, 2.7 mg as daunorubicin and 2.9 mg as the metabolite, daunorubicinol. Initially she was asymptomatic and induction therapy continued with vincristine, 1-asparaginase, prednisone, and IT methotrexate. One week after the daunorubicin injection she developed headache and irritability; CSF protein was 3.2 gm/dl. On the 12th day, she developed fungal sepsis and worsening pneumonia. On the 15th day, she became comatose with a flacid paraparesis, areflexia, and an ascending progressive bulbar palsy. A series of computerized tomography scans over 6 weeks showed increasing diffuse cerebral atrophy. Nerve conduction velocity studies were consistent with an axonal neuropathy. Despite her multiple concurrent medical problems, the timing and characteristics of neurologic damage suggest that IT daunorubicin caused progressive destruction of the nervous system.