In order to clarify our clinical finding that the ATS valve does not open completely, we conducted in vitro tests. Under steady flow conditions, opening of the ATS valve was restricted unless the outlet angle was 0º. Despite this, the pressure gradient of the ATS was similar to that of the St. Jude Medical (SJM) valve, which was found to open fully. Under pulsatile conditions, while the ATS valve opened completely in the ‘straight outlet’, it opened incompletely in the ‘abrupt enlargement outlet’. Substantial turbulent flow was observed at the outside of the leaflets and at the corners of the conduit, particularly with the ATS valve. Compared with the SJM valve, the ATS leaflet axis is located closer to the straight edge, and the leaflets extend further downstream from the ring orifice. These structural features are considered to be the main causes of incomplete opening of the ATS valve.