To clarify the role of cytokines in cerebrospinal fluid (CSF) in the pathogenesis of central nervous system (CNS) leukemia, three cytokine activities, interleukin 1 (IL-1)-beta, tumor necrosis factor (TNF)-alpha, and interferon (IFN)-gamma, and their correlations with other laboratory studies of the CSF were analysed in 23 children with acute leukemia. These patients were classified into three groups: group A (n = 8)--patients with overt CNS leukemia, group B (n = 5)--patients with CNS leukemia in remission, group C (n = 10)--patients without CNS disease. IFN-gamma in the CSF was undetectable in these 23 patients. There was no difference in IL-1-beta levels among the three groups. However, TNF-alpha levels were significantly higher in group A than in group B, and higher in group B than in group C. By Kendall's rank sum test, high TNF levels in CSF correlated with high CSF leukemic cell counts and low sugar levels. In two patients with overt CNS leukemia, the TNF level in the CSF decreased gradually with intrathecal chemotherapy. These results indicate that TNF released from stimulated cells in the cerebrospinal space may induce CNS leukemia-related symptoms or alter laboratory parameters measured in the CSF. TNF levels in CSF may also prove useful in diagnosing early CNS involvement in children with acute leukemia.