Human subcutaneous fat-derived stem cells were recently shown to have the potential to differentiate in vitro into a variety of cell types. Herein it is shown that upon prolonged cell culture fat- derived stem cells spontaneously changed their phenotype from fibroblast-like to cobble-stone appearance which was accompanied by up-regulation of endothelial markers. Undifferentiated fat-derived stem cells displayed long-lasting intracellular calcium oscillations which displayed synchronization in communicating cell clusters. Calcium oscillations were abolished upon inhibition of calcium influx, in calcium-free solution as well as in the presence of La3+ and Li2+. Furthermore, oscillations were abolished in the presence of BAPTA, which chelates intracellular calcium and thapsigargin, which releases calcium from intracellular stores. In summary, our data indicate that fat-derived stem cells spontaneously acquire an endothelial cell phenotype, which may be associated to intracellular calcium fluctuations.