Data of the histological, ultrastructural and culture cell investigations of bone marrow microenvironment in 20 patients with acute lymphoblastic leukemia (ALL) are presented. Morphological and functional peculiarities of the patients' bone marrow stroma different from that in healthy subjects were identified. They included distorted histotopographic patterns of stroma-hemopoietic cell-to-cell interaction, obliteration of sinusoid counts, resorption on the sites of internal bone support structures, a changed stromal cells ratio, formation of reticular cell clusters in the subendosteal and perivascular bone marrow areas, fibroid centers, the presence of nuclear bodies in stromal cell nuclei and an intensified proliferation of stromal cell precursors in organ tissue cultures. Morphological and functional changes in bone marrow microenvironment were detected in ALL patients, which could have been caused both by malignant cell clones and stromal defects.