This article discusses the concept of a common stem cell for bone marrow stromal cells and haematopoietic cells. Until recently it was generally accepted that bone marrow contains two types of stem cells. One is the haemopoietic stem cell; the second one, the mesenchymal stem cell or stromal stem cell, gives rise to the stromal compartment of the marrow. The mesenchymal stem cells can differentiate into osteoblasts, chondroblasts, adipocytes, fibroblasts, and smooth muscle cells. Although the interplay between haemopoietic and stromal cells is well established, no transition of cells from the haemopoietic compartment into cells of the stromal compartment has been demonstrated. Recent data, based on grafting of genetically-marked haemopoietic cells points to the possibility of the generation of adipocytes from haemopoietic stem cells. These findings support the hypothesis postulating a common precursor cell for both bone and bone marrow.There is evidence that osteoblasts can differentiate into adipocytes, and that mesenchymal cells derived from subcutaneous adipose tissue can differentiate into osteogenic cells. The possibility of transdifferentiation of adipocytes into osteoblasts has also been demonstrated.