The clinical and neuropathological investigations have been done on 133 autopsied patients died of leukemia or non-Hodgkin's lymphoma of high malignancy. A study was performed to analyse the role of various factors particularly with respect to adhesion and aggregation in the CNS leukostasis development. The findings were also designated to evaluate the distribution of leukostasis in different CNS regions and to recognize its intensity in various CNS vessels. Basing on the studies the authors conclude that the risk of CNS leukostasis increases evidently when the leukocyte counts are elevated above 50 G/l. The adhesion and aggregation of leukemic and lymphomatous cells as well as local anatomical factors in the CNS vessels play and important role in the CNS leukostasis development, which is more intensive in the white matter and leptomeninges. The medium-sized vessels are much involved, whereas cortical capillaries are relatively less affected by leukostasis. The CNS leukostasis appears to be dynamic and reversible phenomenon, which undergoes fluctuations according to the leukocyte counts increase or decrease.