Insulin-like growth factor-II (IGF-II) is the major IGF in human cerebrospinal fluid (CSF), whereas IGF-I is only detectable in trace amounts. The major IGFBPs in CSF are IGFBP-2 and IGFBP-4. Normally, IGFBP-3 is a minor component in CSF of healthy subjects, but may be increased in pathological states. We investigated IGF-I, IGF-II, and IGFBP-3 levels by specific RIAs in CSF from patients with central nervous system (CNS) tumor or leukemia and compared them with values in patients with meningitis. Further, as proteolysis of IGFBP-3 is part of the modulation of IGF activity, IGFBP-3 fragmentation was quantified by densitometric analysis of [125I]IGFBP-3 protease assays. We examined CSFs of 23 children with malignant CNS tumors, 18 children with leukemia, and 13 children with meningitis. The CSF from 38 children who received lumbar punctures to exclude meningitis was used to define the normal range for IGF-I, IGF-II, IGFBP-3, and IGFBP-3 protease activity in CSF. CNS tumor and leukemia patients had normal levels of IGF-I and IGF-II in CSF, whereas the IGF-II concentration in CSF of meningitis patients was elevated (P < 0.0001). Only 2 of 13 (15%) meningitis patients had elevation of CSF IGFBP-3 concentrations, despite high numbers of inflammatory cells. By comparison, elevated IGFBP-3 concentrations were found in the CSF of 16 of 23 (70%) CNS tumor patients and 6 of 7 (86%) CNS tumor patients with microscopically detectable malignant cells in CSF. Twelve of 13 (92%) patients with medulloblastoma or ependymoma and all 7 medulloblastoma/ependymoma patients with malignant cells in CSF had elevated IGFBP-3 concentrations. The IGFBP-3 protease activity in CSF was elevated in 15 of 16 (94%) patients with CNS tumors of high grade histological malignancy. Five of 6 patients (83%) with acute leukemia and microscopically detectable malignant cells in CSF at the time of diagnosis showed elevated IGFBP-3 concentrations, with normalization after chemotherapy. Leukemia patients without malignant cells in CSF had normal IGFBP-3 concentrations. We conclude that in CSF of children with highly malignant CNS tumor or CNS leukemia, IGFBP-3 is elevated. This phenomenon could be caused by disruption of the blood-CSF barrier and entry of IGFBP-3 from serum, although this appears unlikely, especially for CNS leukemia. More likely possibilities are 1) local production of IGFBP-3 by CNS tumor tissue and secretion into the CSF, or 2) local production of IGFBP-3 by malignant cells within the CSF.