Soft tissue defects resulting from trauma, tumor resection, or congenital causes provide a challenging problem to reconstructive surgery and tissue engineering. Current therapeutic procedures lack the ability to induce rapid formation of neovascularization. Therefore, to date, no adequate application for the reconstruction of soft tissue defects is available. We have previously shown that bioactive factors extracted from adipose tissue (adipose tissue extract [ATE]) induce both adipogenesis and angiogenesis in vitro. These bioactive factors were incorporated into hyaluronan (HA) hydrogel, and the ATE-HA implant-induced angiogenesis and adipogenesis were studied. The developed implant was shown to gradually release the bioactive factors, and the presence of the implant in human adipose stem cell culture was able to induce adipogenic differentiation as evaluated by Oil-red-O staining. In animal experiments, the implants were placed under dorsal subcutis of rodents. Either rat- (rATE, allograft) or human- (hATE, xenograft) derived ATE was incorporated into implants. Local inflammation reactions, angiogenesis, and adipogenesis were followed from 1 week to 40 weeks. Angiogenesis was assessed by microvessel density analysis; adipogenesis was assessed by automated image analysis, and immunological effects by immunostaining and counting inflammatory cells. The key requirements for soft tissue replacement--host compatibility, bioactivity, and sustainability--were all achieved with the novel ATE-HA implant. This acellular implant induced microvessel induction early after implantation and adipose tissue deposition from 12 weeks onward as well as subcutaneous tissue volume increase. The ATE-HA implant was replaced by mature adipose tissue with capillaries, nerve bundles, and healthy connective tissue without local inflammation or capsule formation. The large fat pads remained in tissue until the end of the follow-up time, for 9 months. No adverse effects were detected at the site of implantation, and according to irritating ranking, the ATE-implant was considered to have excellent biocompatibility. The results demonstrate that an acellular HA hydrogel implant induces significant increase in adipogenesis and angiogenesis in vivo compared to the plain HA implant, and ATE has excellent potential for use in tissue engineering for sustained reconstruction of soft tissue defects.