Background. Currently, there is a tendency to use large-size prosthetic valves in mitral position in order to achieve better hemodynamic performance. We aimed to evaluate hemodynamic performance of 25 mm prosthetic valves in mitral position. Methods: Between August 1998 and May 2000, a total of 40 patients, 34 women and 6 men (mean age 42 12 years), underwent mitral valve replacement with 25 mm bileaflet mechanical valve. Preoperative functional capacity was New York Heart Association classes III and IV in 31 patients. Major indication for surgery was severe mitral stenosis in all patients. Results: Total hospital mortality was 5% (two patients). Mean follow-up period was 14.7 +/- 4.9 months (2 to 23 months). No thromboembolic event, paravalvular leak, mechanical failure, endocarditis were observed during the follow-up period. Postoperatively 92.5% of the patients were in the functional capacity of New York Heart Association class I. Postoperative echocardiographic observations have given an average pulmonary artery pressure of 29 +/- 2 mmHg, effective orifice area as 2.95 +/- 0.1 cm(2), and mean prosthetic gradient as 6.5 +/- 2.7 mmHg. Conclusion: In short- and mid-term, small-size (25 mm) Sorin mechanical mitral valves had excellent hemodynamic performance in patients with severe mitral stenosis, moderate or serious pulmonary hypertension, and small mitral annuli, whose body surface area was smaller than 1.6 m(2).