The in vivo echocardiographic behavior of a new mitral allograft design that incorporates ePTFE patches and chordae to enhance durability has been evaluated in sheep. A strip of pericardium was sewn around the anulus margin and an ePTFE patch of 0.4 mm thickness placed over the truncated papillary muscle tip. In four out of ten cases, 5/0 ePTFE sutures were used additionally to reinforce the native chordae of the anterior leaflet. The allografts were cold-stored at 4 degrees C after antibiotic treatment and implanted in weanling sheep. Valvular function was first examined intraoperatively by transesophageal echocardiography and again after a mean follow up of 153 +/- 8 days by epicardial echo. In all six long term survivors, valve motion was unimpaired and similar to that of the natural mitral valve. There was sufficient coaptation and concentric opening action of the leaflets. Ventricular contraction was enhanced by maintaining the annulo-ventricular connection of the native valve. Color and pulsed Doppler revealed a physiologic biphasic flow pattern and only minor regurgitation in one case. Mean transvalvular pressure gradient was 3.3 +/- 2.1 mmHg on average which correlates well with values obtained from subsequent cardiac catheterization. In conclusion, this new design of a mitral allograft shows excellent in vivo echocardiographic behavior after five months of implantation. Therefore, human allografts of this type could be recommended for clinical application.