Adipocytes and perivascular adipose tissue are emerging to regulate vascular functions. Effects of adipocytes and perivascular adipose tissue on human smooth muscle cell (SMC) proliferation were investigated. Conditioned medium was prepared from cultured premature and differentiated 3T3-L1 adipocytes, from peri-aortic adipose tissue from young (3 months) and old (24 months) WKY rats, from lean and obese Zucker rats (3 months), and from WKY rats fed normal chow or a high fat diet for 3 months. Conditioned medium from differentiated (but not premature) adipocytes stimulated SMC proliferation, which was abolished by charcoal and proteinase K treatment, but resistant to heat, trypsin or phospholipase B (to hydrolyze lysophosphatidic acid). Further experiments demonstrated that the growth factor(s) is hydrosoluble, present in the fraction of molecular weight >100 kDa. Moreover, conditioned medium from peri-aortic adipose tissue stimulated SMC proliferation, which was significantly enhanced in aged rats and in rats fed high fat diet, but not in obese Zucker rats deficient in leptin receptor. In conclusion, mature adipocytes release a hydrosoluble protein growth factor(s) with molecular weight >100 kDa for SMC. Perivascular adipose tissue stimulates SMC proliferation, which is enhanced in aged WKY and in high fat diet-induced obesity but not in leptin receptor deficient obese Zucker rats. This adipocyte-derived growth factor(s) and the effect of perivascular adipose tissue may be involved in vascular disease associated with aging and obesity.