White adipose tissue (WAT) is the main type of adipose tissue in human adults. Whether white adipocyte progenitors derive only from permanently WAT-residing cells or whether cells migrating from other tissues may contribute to adipogenesis is unclear. Rydén et al. investigated subcutaneous WAT from 65 patients who had undergone allogeneic transplantation with either whole bone marrow (BM) or mobilized peripheral blood stem cells (PBSCs). By taking advantage of genomic differences between donor and recipient cells, and using mathematical modelling, they estimated that BM- or PBSC-derived progenitors contribute ∼10% to the adipocyte population over the entire lifespan. The contribution was up to 2.5 times higher in obese patients, indicating that the increased need for adipocytes in obesity can be met, in part, by BM- or PBSC-derived progenitors. Exome and whole-genome sequencing of single adipocytes suggested that these progenitors incorporate into WAT by cell differentiation and cell fusion. However, single-cell fate studies to provide direct proof for such processes cannot be performed.