The Dutch Björk-Shiley convexo-concave (BScc) cohort serves as a reference population on the risk of outlet strut fracture and is being used to formulate guidelines for prophylactic replacement. Fractures, however, may be undetected at death. The aim of this study was to quantify the degree of underestimation of strut fracture in the Dutch BScc cohort. Multivariate Cox regression analysis was used to assess the relative and absolute risk of death from different causes within 14 years. The unexplained "excess" mortality among 70 degrees BScc valve recipients was attributed to unreported fatal strut fractures and used to estimate its extent in this group, which then was extrapolated to the 60 degrees BScc valve recipients. For 70 degrees BScc valve recipients, the adjusted hazard ratio for death from all causes except strut fracture was 1.2 (95% confidence interval [CI] 1.0 to 1.5). The 14-year absolute risks for 70 degrees and 60 degrees BScc valve recipients were 44% and 37%, respectively. Among 70 degrees and 60 degrees BScc valve recipients, underreporting of fracture was estimated to be 25% (95% CI 0 to 49) and 26% (95% CI 0 to 52), respectively. Estimates based on sudden death and fatal congestive heart failure yielded essentially the same results. Thus, underreporting of fatal strut fracture in the Dutch BScc cohort is estimated to be approximately 25%. Hence, the risk and lethality of fracture of BScc valves are underestimated and indications for prophylactic replacement should be adjusted accordingly. For example, the advantage of valve replacement in a 40-year-old patient with a 29-mm 60 micro BScc mitral valve would almost double to 0.82 years.