To assess the regurgitant characteristics of mitral biologic and mechanical prostheses immediately after implantation, intraoperative transesophageal echocardiography was performed in 27 patients, aged 32 to 69 years, undergoing open-heart surgery for rheumatic heart disease (n = 19), mitral valve prolapse (n = 3), malfunctioning prostheses (n = 3), or periprosthetic leaks (n = 2). The prostheses included 13 biologic (Carpentier-Edwards) and 14 mechanical valves (five Starr-Edwards, five Medtronic-Hall, and four Bjork-Shiley). Physiologic transvalvular regurgitant flow was detected in both biologic and mechanical prostheses. The spatial extent of the regurgitant jets was usually greater in the mechanical than in the biologic valves, and systolic jets, characteristic of each type of valve, were visualized consistently. Trivial periprosthetic jets (PPJs) were observed in many implanted valves (14/27). The median maximal jet area was 0.46 cm2 (range 0.1 to 1.5 cm2). Cardiopulmonary bypass was reinstituted in two patients. In one patient a PPJ was judged extensive enough (area 3.6 cm2) to warrant surgical revision of the implant, but no dehiscence was found. In the other patient a turbulent PPJ (area 5.5 cm2) was associated with a 0.5 cm dehiscence at the surgical inspection. In conclusion, (1) all mitral prostheses exhibit physiologic transvalvular regurgitation, (2) trivial mitral PPJ is a common finding in newly implanted mitral valves and does not require the revision of the implant, and (3) further experience based on larger series of patients is required to determine the maximal acceptable size of a mitral PPJ detected by intraoperative transesophageal echocardiography.