Over the past 20 years, the use of biological prostheses has increasingly been favoured over the use of mechanical prostheses for both aortic and mitral valve replacement, despite a lack of evidence to support this preference. Goldstone et al. reviewed the long-term outcomes of patients who received a prosthetic valve in California, USA, between 1996 and 2013. In 9,942 patients who underwent aortic valve replacement, use of a biological valve was associated with higher 15-year mortality than use of a mechanical valve in patients aged 45–54 years (HR 1.23), but not in those aged 55–64 years. In 15,503 patients who underwent mitral valve replacement, use of a biological valve was associated with higher 15-year mortality than use of a mechanical valve in patients aged 40–49 years (HR 1.88) and in those aged 50–69 years (HR 1.16). Reoperation rates were higher with a biological valve, but mechanical valves were associated with higher rates of bleeding and, in some age groups, stroke.