The incidence of and factors that predispose to outlet strut fracture of Björk-Shiley heart valves are still not known. To obtain such information a retrospective cohort study was conducted on all 2303 patients in the Netherlands with a 60 degrees convexo-concave (60 degrees CC) or a 70 degrees convexo-concave (70 degrees CC) Björk-Shiley heart valve. Patients have been followed-up for a mean of 6.6 years (range 1-4271 days). 42 cases of mechanical failure due to outlet strut fracture have been recorded-6 of the 7 patients with fracture of the aortic valve died, as did 18 of the 35 patients with fracture of the mitral valve. Multivariate analysis identified wide opening angle (70 degrees), large valve size (greater than or equal to 29 mm diameter), and young age (less than 50 years) as risk factors for outlet strut fracture. For large 70 degrees CC mitral valves the cumulative risk of outlet strut fracture after 8 years was 17.4% (95% CI 9.1-31.6). Unlike previous findings, this excessive risk applied to late as well as to early batches of valves. In patients with a large 60 degrees CC mitral valve the cumulative risk after 8 years was 4.2% (95% CI 2.7-6.5). The incidence rate of outlet strut fracture in 60 degrees CC and 70 degrees CC valves (aortic and mitral) was constant over time. Overall survival since implantation was better for patients with 60 degrees CC prostheses than for those with 70 degrees CC prostheses; the adjusted hazard ratio for mortality for patients receiving a 70 degrees CC prosthesis was 1.5 (95% CI 1.1-2.0). Together with the low (24%) necropsy rate, this ratio suggests that the reported incidence of strut fracture for the 70 degrees CC valves is an underestimate. The data indicate that prophylactic replacement of 60 degrees CC and 70 degrees CC valves is advisable for selected groups of patients. Since the case-fatality rate is 50% for emergency replacement of faulty valves, patients suspected of Björk-Shiley heart-valve failure should be referred without delay to a cardiothoracic centre.