BACKGROUND: Selection of biological versus mechanical heart valve replacement entails tradeoffs in the risk and expected cost of post-operative bleeding, embolic events and reoperation. Additionally, the risk of these events varies by age at implant. OBJECTIVE: To identify the direct medical costs to the NHS of biological and mechanical valve replacement in younger versus older patients. METHODS: A Markov decision-analytic model was constructed to identify the cumulative lifetime costs of valve replacement and related events in a simulated cohort of 10,000 patients followed from valve implantation until death. Events included bleeding, embolism, endocarditis, structural valve deterioration, reoperation and death. Event rates were modeled using linear, and non-linear statistical hazard functions based on clinical series reported in the literature. Medical resource use related to events was estimated based on clinical expert opinion. Costs were assigned to each event using standard lists of NHS costs. RESULTS: For aortic valve replacement, the expected lifetime costs were £ 6,812 (biological) and £ 8,873 (mechanical) for persons aged 60, versus £ 6,281 (biological) and £ 8,137 (mechanical) for persons aged 70 at implant, respectively. In the mitral position, costs were £ 6,968 (biological) and £ 8,760 (mechanical) versus £ 6,299 (biological) and £ 7,989 (mechanical) in persons aged 60 versus 70 at implant respectively. Results were most sensitive to bleeding, embolic and reoperation event rates, but less sensitive to the cost per event. CONCLUSION: The expected lifetime cost of biologic valve replacement was lower than mechanical valve replacement for both age groups and valve positions. This suggests the economic impact of anticoagulation therapy, bleeding and embolic events, which occur at higher rates in the mechanical valve, is greater than the economic impact of structural valve deterioration leading to reoperation, which is greater in the biological valve.