Morphological observations have long shown the highly plastic nature of bone marrow adipous tissue and its possible relations with hematopoiesis and its bony environment. Cell cultures with murine stromal cell lines or cells directly issued from human marrow confirm these interactions and demonstrate that 1) in the bone marrow cavity, hematopoietic progenitor cells coexist with mesenchymal stem cells which can produce adipocytes and osteoblasts ; 2) depending on the degree of adipogenesis, the bone marrow microenvironment could locally encourage either lymphopoiesis or myelopoiesis ; 3) in addition to their role as an energy reserve, adipocytes are endocrine cells which secrete cytokines and growth factors involved in bone remodeling and hematopoietic activity. In particular, bone marrow adipocytes secrete large quantities of leptin, which appears to play a part in the regulation of CD34+ cells and their differentiation into granulocyte and monocyte progenitors. The local action of the leptin produced by the marrow adipocytes may be completed by that produced by adipous tissues other than bone marrow. This has been observed in obese subjects and during chronic inflammation, situations characterized by high plasma levels of leptin and frequent hyperleukocytosis. Bone marrow adipous tissue should no longer be considered simply as a filling material but as an adaptable tissue with marked metabolic and secretory activity, involved in the regulation of hematopoiesis and its environment.