Observations among Karlsburg patients in 2006 revealed that the majority of very low platelet levels inducing postoperative heparin-induced-thrombocytopenia (HIT)-diagnostics with at the end negative results appeared related to aortic valve replacement (AVR) with stentless bioprostheses. We compared the postoperative courses of platelet counts in patients having had AVR with stentless prostheses (Sorin Biomedica Freedom Solo [SOLO]) or stented prostheses (Carpentier Edwards Perimount [PM]). Between February 2005 and April 2007, 209 patients received AVR with SOLO, in 137 patients a PM-prosthesis was implanted. The mean platelet levels were compared from the first up to the fifth postoperative day. A higher occurrence of platelet levels below 100 Gpt/l between the second and the fifth postoperative day was found in the SOLO-group (71.9%) compared with the other biological substitute PM (36.6%). Differences in platelet counts between SOLO- and PM-subgroups were measured for day 2 (P=0.03), day 3 (P=0.0004) day 4 (P=0.0007), day 5 (P=0.0002) and at discharge (P<0.0001). Following intervention with conventional biological AVR, differences in the postoperative recovery of platelet counts can be detected, depending on the prosthesis used. The causes for and the clinical implications of this phenomenon are not yet assessed.