Involvement of CNS with leukemic cells is well recognized complication of acute lymphatic leukemia (ALL) in childhood, but with recent improvements in systemic treatment and longer survival the incidence of this complication has increased in adults. Neurological symptomatology in patients with CNS leukemia is due to meningeal infiltration, but sometimes also to diffuse and nodular cerebral infiltration. Between January 1991 and December 1994, 36 patients suffering of acute leukemia, 28 with ALL, and 8 with acute myeloid leukemia (AML) were demonstrated to have neuroleukemia by the following criteria: (1) the presence at lumbar puncture (LP) pleocytosis and blast cells on CSF sediment (without positive bacteriologic and fungal cultures), and (2) the presence of neurological symptoms and signs. All 36 patients had 46 episodes of CNS involvement. All patients had neurological examinations during every episode, and according to the neurological abnormalities were classified into four categories. LP was performed in all, and CSF sediments obtained by sedimentation in Sayk's chambers, were routinely stained by MGG and cytochemical stains to detection of leukemic cells (Fig. 1). EEG was done during 21 episodes, CT scan during 15. We divided patients into four groups according to the most prominent neurological symptoms and signs. First was the group included 23 episodes (50%), (18 ALL, 5 AML), where symptoms and signs of meningeal irritation predominated, mimicking the clinical picture of meningitis. This meningeal syndrome can sometimes produce differential diagnostic problems with CNS infections, when CSF examination is of primary importance. Second was the group of 9 patients (6 ALL and 3 AML) with 10 episodes (21.74%) where cranial nerve symptoms and signs-predominated, or were exclusively present. Most frequently affected were bulbomotors, facials and opticus. Third group consisted of 8 ALL patients (8 episodes, 17.39%) with dominant spinal root symptomatology, caused by pathological infiltration of either spinal roots or meninges surrounding them. This group includes also one patient with mononeuritis multiplex and the other with painful polyneuropathy. All patients in this group had pain on straight leg raising, but we stress here that all patients from other groups had positive Lazarevitsh's sign, too. So, it can be a good differential diagnostic parameter for distinguishing toxic medicamentous polyneuropathy from leukemic poliradiculoneuropathy. Fourth group included 5 patients (5 episodes, 10.87%). 4 ALL and 1 AML, where cerebral symptoms, such as seizures, hemiparesis and psychoorganic syndromes were prominent. CSF was obtained during all episodes by lumbar puncture. The protein concentration ranged from 21-3180 mg/dl, and was above normal (45 mg/dl) during 28 episodes. Mild hypoglycoracchia was present during 16 episodes. Cell count ranged from 11-4816 cells/cm3, malignant cells were identified during all episodes with same morphological and cytochemical characteristics of identified type of leukaemia. It has been established that the most valuable diagnostic procedure in CNS leukemia is CSF examination, and detection of blasts is sufficient for diagnosis. All other procedures like EEG, myelography and CT have only supplemental diagnostic significance. Finally, in this study we showed that neurological symptomatology in patients with acute leukemia is not dependent of the type of leukemia, moreover different types of AL can have same neurological manifestations. As others, we sometimes used the term CNS leukemia in this paper, although it is clear that meninges and peripheral nervous system are most often involved. This is the reason why we suggest that neuroleukemia, or NS leukemia should be used as more appropriate expressions.