Autologous fat grafting is commonly used for soft tissue augmentation, but its unpredictably high resorption rate remains a major limitation. Although adipose-derived mesenchymal stem cells (ASCs) are an attractive candidate for enhancing graft retention, their poor posttransplantation viability limits their application. The authors aimed to evaluate the effect of incubated ASCs on microfat graft survival in an immunocompromised mouse model. Lipoaspirates for microfat injection were collected from the wasted lower abdominal adipose tissues of 5 patients who had undergone breast reconstructive surgery with an abdominal flap. Adipose-derived mesenchymal stem cells were also isolated and proliferated from these fat tissues. Sixty athymic mice were randomly allocated to a control group (microfat grafting alone; n = 30) or ASCs group (microfat grafting plus simultaneous human ASCs injection; n = 30). The volume and weight of survived fat were measured at 8 and 16 weeks, and histopathological and immunologic staining was performed at 16 weeks. The survived fat volume of the ASCs group was significantly greater than that of the control group at 8 and 16 weeks, whereas the weight of survived fat tissues did not significantly differ. Histologic evaluation of the harvested fat indicated significantly higher levels of adipocytes, and fewer cysts and fibrosis in the tissues in the ASCs group than in the control group. The ASCs group also exhibited a significantly higher number of capillary vessels than the control group on CD31 and alpha-smooth muscle actin staining. In conclusion, transplanted fat survival is markedly higher when simultaneous microfat graft and ASCs injection were performed, as compared with that in the classical microfat graft alone method in mice; this improvement was primarily attributed to the increased ability to produce blood vessels.