Concern has been raised regarding the long-term durability of the Mitroflow biological heart valve prosthesis. Our aim was to assess the incidence of structural valve degeneration (SVD) for the Mitroflow bioprosthesis in a nationwide study in Denmark including all patients alive in Denmark who had received a Mitroflow aortic bioprosthesis since 2000.Patients alive in Denmark with a Mitroflow bioprosthesis implanted since January 2000 were invited to participate in a nationwide cross-sectional study with a predefined definition of SVD. Of 1552 patients, 861 patients had died and 47 patients had been reoperated with 40 reoperations due to SVD. The remaining 644 patients were invited for evaluation; 574 patients accepted and were evaluated for SVD. The incidence of SVD was calculated using competing risk regression analysis with death as the competing event.A total of 173 patients were diagnosed with SVD by echocardiography. Of these, 64 (11%) patients had severe SVD and 109 (19%) patients moderate SVD. Severe SVD was associated with the age of the prosthesis and small prosthesis size [Size 21: hazard ratio (95% confidence interval, CI) 2.72 (0.97-8.56), P = 0.06; Size 19: 6.26 (1.63-24.06), P = 0.008]. The cumulative incidences of reoperation or severe SVD at Year 9 were 12.5% for Size 19, 7.6% for Size 21 and 3.1 (1.2-6.4)% for Size 23. Median survival in patients with prosthesis Sizes 23-29 was 6.4 (95% CI 5.7-7.0) years, with Size 21 it was 6.5 (95% CI 5.9-7.1) years and with Size 19 it was 6.9 (95% CI 5.7-8.2) years (P = 0.78).The incidence of undetected severe SVD was as high as the incidence of operated SVD. The overall risk for SVD is high for the Mitroflow bioprosthesis, especially if the prosthesis is small and older than 5 years.