Background Autologous fat grafting is increasingly used in reconstructive surgery. However, resorption rates ranging from 25% to 80% have been reported. Therefore, methods to increase graft viability are needed. Here, we report the results of a triple-blind, placebo-controlled trial to compare the survival of fat grafts enriched with autologous adiposederived stem cells (ASCs) versus non-enriched fat grafts. Methods Healthy participants underwent two liposuctions taken 14 days apart: one for ASC isolation and ex-vivo expansion, and another for the preparation of fat grafts. Two purified fat grafts (30 mL each) taken from the second liposuction were prepared for each participant. One graft was enriched with ASCs (20 ×106 cells per mL fat), and another graft without ASC enrichment served as a control. The fat grafts were injected subcutaneously as a bolus to the posterior part of the right and left upper arm according to the randomisation sequence. The volumes of injected fat grafts were measured by MRI immediately after injection and after 121 days before surgical removal. The primary goal was to compare the residual graft volumes of ASC-enriched grafts with those of control grafts. This study is registered at www.clinicaltrialsregister.eu, number 2010-023006-12. Findings 13 participants were enrolled, three of whom were excluded. Compared with the control grafts, the ASCenriched fat grafts had significantly higher residual volumes: 23·00 (95% CI 20· 57-25· 43) cm3 versus 4· 66 (3· 16-6· 16) cm3 for the controls, corresponding to 80· 9% (76· 6-85· 2) versus 16· 3% (11· 1-21· 4) of the initial volumes, respectively (p<0· 0001). The difference between the groups was 18· 34 (95% CI 15· 70-20· 98) cm3, equivalent to 64· 6% (57· 1-72· 1; p<0· 0001). No serious adverse events were noted. Interpretation The procedure of ASC-enriched fat grafting had excellent feasibility and safety. These promising results add significantly to the prospect of stem cell use in clinical settings, and indicate that ASC graft enrichment could render lipofilling a reliable alternative to major tissue augmentation, such as breast surgery, with allogeneic material or major fl ap surgery.