A complex study of colony-stimulating properties of cells-precursors of granulomonocytopoiesis, morphological parameters of the marrow histological structures and immunological parameters was carried out in 90 patients to specify pathogenic mechanisms of aplastic anemia (AA) development. 95% of patients appeared to have sharp decrease of the marrow colony-stimulating properties and only 5% had normal initial indexes. In most patients endostal cell number increased, while the content of reticulocytes grew higher in more than 30% of patients. Defect of the marrow stroma functioning was registered in 23% of patients. Increase of the osseous tissue volume and osteocyte number was found. Dysbalance in cellular link of immunity was demonstrated. Only temporal increase of the marrow colony-stimulating properties was marked in some patients on the background of treatment with antilymphocyte globulin, despite the clinic-hematological improvement. Multifactor pathophysiological mechanisms, the main of which is the stem cell defect lies in the base of AA pathogenesis. Hemopoietic microenvironment pathology is not the leading reason of the hemopoiesis depression but it contributes greatly in its development. Immunological alterations in patients with AA are secondary and can be partly corrected with treatment with ALG.