Autologous fibrin-based tissue-engineered heart valves have demonstrated excellent potential as patient-derived valve replacements. The present pilot study aims to evaluate the structure and mechanical durability of fibrin-based heart valves after implantation in a large-animal model (sheep). Tissue-engineered heart valves were molded using a fibrin scaffold and autologous arterial-derived cells before 28 days of mechanical conditioning. Conditioned valves were subsequently implanted in the pulmonary trunk of the same animals from which the cells were harvested. After 3 months in vivo, explanted valve conduits (n = 4) had remained intact and exhibited native tissue consistency, although leaflets demonstrated insufficiency because of tissue contraction. Routine histology showed remarkable tissue development and cell distribution, along with functional blood vessel ingrowth. A confluent monolayer of endothelial cells was present on the valve surface, as evidenced by scanning electron microscopy and positive von Willebrand factor staining. Immunohistochemistry and extracellular matrix (ECM) assay demonstrated complete resorption of the fibrin scaffold and replacement with ECM proteins. Transmission electron microscopy revealed mature collagen formation and viable, active resident tissue cells. The preliminary findings of implanted fibrin-based tissue-engineered heart valves are encouraging, with excellent tissue remodeling and structural durability after 3 months in vivo. The results from this pilot study highlight the potential for construction of completely "autologous" customized tissue-engineered heart valves based on a patient-derived fibrin scaffold.