Cerebrospinal fluid (CSF) neopterin levels were determined by high-pressure liquid chromatography in 48 normal children and in 15 children with meningeal relapse of hematologic malignancies (13 acute lymphoblastic leukemia and 2 high-grade lymphomas). When meningeal relapse was diagnosed, all patients had CSF neopterin levels higher than mean normal value +2 standard deviations. No significant correlation between the blast count in the CSF and neopterin levels was observed. CSF data before relapse were available in 10 children: the neopterin values at relapse were significantly higher than values observed at diagnosis. In 3 patients, elevated neopterin levels preceded the occurrence of neurologic signs and the detection of blast cells in CSF by 15 to 30 days. In the absence of infection, the rise of CSF neopterin levels in patients with hematologic malignancies indicates an active phase of the disease. This could reflect a cell-mediated immunologic process induced by malignant cells. The measurement of CSF neopterin should be helpful in the monitoring of patients to detect early meningeal relapse.