Levels of Thymidine Kinase (TK) in cerebrospinal fluid (CSF) were longitudinally evaluated in 68 patients with acute leukemia. Values significantly above the normal range were found in all cases with meningeal involvement (in two subjects before the cytological evidence of leukemic cells in CSF). Such high levels decreased until normal values after an effective treatment. Two patients with multiple cerebral neoplastic nodules without infiltration of meninges showed normal CSF-TK values. No false positive cases were observed. Both kinetic activity and mass of leukemic cells appeared to influence CSF-TK levels. Thus, CSF-TK appears to be a promising marker in diagnosing and monitoring meningeal disease in the course of acute leukemia.