In 38 patients with acute leukemias or non-Hodgkin's lymphomas of high malignancy the blood-brain barrier (BBB) was prospectively analysed by means of QAlb value (QAlb = cerebrospinal fluid albumin/serum albumin). Cerebro-spinal fluid and serum were taken before each intrathecal methotrexate administration according to central nervous system (CNS) prophylaxis or treatment of CNS involvement by neoplasm. Patients were divided into two groups. Group I consisted of 5 individuals with clinical manifestations of CNS involvement by leukemia or lymphoma. Group II contained 33 patients without neurological symptoms. Besides, group II was subdivided into other two groups: group IIa-patients with BBB analysis before cytostatics application, and group IIb-patients in which BBB was analysed after the administration of at least one cycle of protocols, used in general chemotherapy, including intrathecal methotrexate injection. In group I BBB changes were observed in 10 out of 12 assessments (sensitivity 83.3%). In group II BBB impairment was revealed in 11 out of 52 assessments (specificity 78.8%). The differences in QAlb values between groups I and II were statistically significant (p = 0.0008), whereas there were no significant differences between QAlb values in groups IIa and IIb. Basing on their investigations, the authors conclude that neoplasm invading CNS should be considered as essential risk of BBB impairment, whereas intrathecal and general chemotherapy appear to be less important in BBB injury.