Little is known about whether bone marrow–derived circulating progenitor cells (BMDCPCs) can transdifferentiateinto adipocytes in adipose tissues or play a role in expanding adipocyte number during adiposetissue growth. Using a mouse bone marrow transplantation model, we addressed whether BMDCPCs cantransdifferentiate into adipocytes under standard conditions as well as in the settings of diet-induced obesity,rosiglitazone treatment, and exposure to G-CSF. We also addressed the possibility of transdifferentiation toadipocytes in a murine parabiosis model. In each of these settings, our findings indicated that BMDCPCs didnot transdifferentiate into either unilocular or multilocular adipocytes in adipose tissues. Most BMDCPCsbecame resident and phagocytic macrophages in adipose tissues — which resembled transdifferentiated multilocularadipocytes by appearance, but displayed cell surface markers characteristic for macrophages — inthe absence of adipocyte marker expression. When exposed to adipogenic medium in vitro, bone marrow cellsdifferentiated into multilocular, but not unilocular, adipocytes, but transdifferentiation was not observed invivo, even in the contexts of adipose tissue regrowth or dermal wound healing. Our results suggest that BMDCPCsdo not transdifferentiate into adipocytes in vivo and play little, if any, role in expanding the number ofadipocytes during the growth of adipose tissues.