At least 2 different types of cells, hematopoietic and mesenchymal, are present in the adult bone marrow, in addition to endothelial cells. Hematopoietic and mesenchymal cells are believed to originate from hematopoietic stem cells (HSC) and mesenchymal stem cells (MSC), respectively. The bone marrow stroma, a cellular microenvironment that supports HSC, is composed of non-hematopoietic cells and contains MSC. A unique expansion of the bone marrow stroma, also known as marrow fibrosis, is the hallmark of a variety of disorders including hyperparathyroidism and fibrous dysplasia. PTH is the first bone anabolic agent approved by US Food and Drug Administration for the treatment of osteoporosis. Recent studies have suggested that PTH treatment may affect the number of hematopoietic stem cells in the bone marrow and their mobilization into the bloodstream. In addition, cells with classical features of mesenchymal stem cells/progenitors have been shown to express receptors for PTH, and to increase in number and undergo redistribution in the adult bone marrow upon PTH treatment. In this review, we will summarize the up-to-date knowledge on PTH and its relation to stem cells. We will also discuss the contribution of different cell types to the development of marrow fibrosis and the involvement of PTH signaling in this pathology.