A disorder involving the aberrant infiltration and aggregation of leukocytes into the vasculature of the body. Leukostasis is typically detected in the brain and lungs of persons with leukemia. It requires substantial ablative modalities to both reduce the number of cells present and to ensure dispersion of the aggregates. The disease is leukostasis (MONDO_0006831). Also known as: leukostasis (morphologic abnormality).