Stentless bioprostheses have been considered to achieve superior hemodynamics over stented bioprostheses for aortic valve replacement with improved long-term performance. We observed severe thrombocytopenia in patients who received the Sorin Freedom Solo aortic stentless pericardial bioprosthesis within the first days after implantation. Absolute and relative platelet counts within 2 weeks after implantation of either a stentless (Sorin Freedom Solo) or a stented (Sorin Mitroflow) bovine pericardial bioprosthesis were compared in a matched-pairs analysis in 40 patients. Except the preoperative values, absolute platelet count was higher at all time points in the Mitroflow group. In the Mitroflow group, the mean platelet count moderately dropped to a minimum of 60% of the initial value on POD 3 and fully recovered by POD 8. In the Freedom Solo group, platelet loss was significantly more severe (minimum relative value 25% on POD 4) with no recovery during follow-up (60% on POD 13). Eight patients of the Freedom Solo group experienced a critical platelet drop towards <20% of their initial values, in five of them absolute numbers decreased below 30,000/microl. No bleeding complications or other morbidity occurred. Attention should focus on the platelet count after implantation of the Freedom Solo bioprosthesis, especially in patients who are supposed to receive platelet inhibitors. However, the described phenomenon remains unexplained.