The objective of this prospective 18-month study was to evaluate the clinical and laboratory effects of repeated intrathecal injections of chemotherapy in children with acute leukemia. All procedures were performed under general anesthesia, and complications were prospectively recorded. Laboratory measurements included lumbar puncture opening pressure, cerebrospinal fluid (CSF) chemistry, and cell count and morphology. Central venous pressure and ophthalmologic examinations were also performed. Forty-seven children underwent 247 intrathecal injections of chemotherapy. Adverse effects (13.7% of the procedures) included nausea and vomiting, back pain, and headache. One child each had transient cauda equina syndrome, transient communicating hydrocephalus, and persistent sacral plexus injury. The mean lumbar puncture opening pressure was significantly higher after intrathecal therapy than before (22 +/- 8 vs. 15 +/- 9 cm H2O, P = 0.02) and higher than reported in age-matched children without leukemia. All CSF chemistries, cell count, and morphology were normal. The overall incidence of complications was 13.7%. Most were mild and resolved quickly, but significant neurologic complications did occur. Lumbar puncture opening pressure was significantly higher in children with acute leukemia after intrathecal chemotherapy.