Vascularization of engineered tissues is one of the current challenges in tissue engineering. Several strategies aim to generate a prevascularized scaffold which can be implanted at sites of injury or trauma. Endothelial cells derived from peripheral blood (outgrowth endothelial cells, OECs) display promising features for vascular tissue engineering, including their autologous nature, capacity for proliferation and ability to form mature vessels. In this study we investigated the ability of OECs to form vascular structures in co-culture with adipose-derived stem cells (ASCs) in a fibrin matrix. Using microcarrier beads coated with OECs, we showed ingrowth of endothelial cells in the fibrin scaffold. Furthermore, co-cultures with ASCs induced vessel formation, as evidenced by immunostaining for CD31. The degradation of fibrin is at least in part mediated by expression of matrix metalloproteinase-14. Moreover, we showed OEC/ASC-induced vessel-like structure formation even in the absence of microcarrier beads, where increasing amounts of ASCs resulted in a denser tubular network. Our data add new insights into co-culture-induced vessel formation of outgrowth endothelial cells within a fibrin matrix in an autologous system.