Technical Abstract: Increases in adipocyte number represent a major contribution to the expansion in adipose tissue mass characteristic of obesity induced by either diet, genetics or hormones in man and animals. The identity and regulation of the adipocyte precursor cell, or preadipocyte, and the preadipocyte precursor cell, or progenitor cell, have been intensely studied for many years. In this issue of JCL, Crossno et al. reports that an increase in adipocyte number may also be attributable to non-resident progenitor cells. They showed that either rosiglitazone treatment or a high fat diet promoted the trafficking of circulating bone marrow-derived progenitor cells into adipose tissue where they became multilocular adipocytes. This adds a new and unexpected dimension to this research arena.