In children, systemic heart valve replacement with bioprostheses is associated with accelerated valve degeneration, and mechanical prostheses require permanent anticoagulation. Novel "biomechanical" polymeric valve prostheses ("bio" = flexible, "mechanical" = synthetic), solely made of polycarbonate urethane (PCU), were tested in vitro and in a growing animal (calf) model with the aim of improved durability without permanent anticoagulation. The trileaflet aortic prosthesis has diminished pressure loss and reduced stress and strain peaks. The asymmetric bileaflet mitral valve mimics natural nonaxial inflow. The valves underwent long-term in vitro testing and in vivo testing in growing calves for 20 weeks [mitral (7), aortic (7)] with comparison to different commercial bioprostheses [mitral (7), aortic (2)]. In vitro durability of PCU valves was proved up to 20 years. Survival of PCU valves versus bioprostheses was 7 versus 2 mitral and 5 versus 0 aortic valves, respectively. Two animals with PCU aortic valves died of pannus overgrowth causing left ventricular outflow tract obstruction. Degeneration and calcification were mild (mitral) and moderate (aortic) in PCU valves but were severe in biological valves. There was no increased thrombogenicity of the PCU valves compared to bioprostheses. The novel polymeric valve prostheses revealed superior durability compared to current bioprostheses in growing animal model without permanent anticoagulation and thus, may be a future option for pediatric patients.