Echocardiography plays a critical role in assessing prosthetic valve endocarditis. Because normal paravalvular findings can mimic paraprosthetic infection early after implantation of a stentless bioprosthesis, we sought to define echocardiographic characteristics associated with infective endocarditis (IE) complicating stentless tissue aortic valve replacement. Between September 1992 and October 2000, 388 patients underwent aortic valve replacement with a Freestyle stentless tissue aortic valve. Nine patients presented with clinical endocarditis 10 days to 107 weeks after surgery. Patients included 8 men and 1 woman, ages 38 to 72 years. Of these, 7 patients underwent valve explantation, 1 patient was treated medically, and 1 died within hours of presentation. Intraoperative post-pump transesophageal echocardiography (TEE) and subsequent TEE examinations were reviewed for pertinent findings. For comparison, 22 patients without IE who underwent follow-up TEE within 1 year after Freestyle aortic valve replacement served as a control group. Abnormal TEE findings in patients with IE included new or worsening paravalvular aortic regurgitation (AR) in 4, diffuse leaflet thickening in 4, valvular vegetations in 1, and aorto-atrial fistula in 1. A progressive increase in the paravalvular echo-dense and/or echo-lucent space occurred in 5 of 9 patients. Among control subjects, paravalvular findings observed on immediate post-pump TEE resolved over time, and did not increase in size in any patient. In addition, no control patient developed new or progressive AR, diffuse leaflet thickening, or vegetations. TEE is useful in detecting valvular and paravalvular involvement of IE complicating stentless tissue aortic valve replacement. Because incremental change in paravalvular appearance from post-pump TEE is an important finding, intraoperative post-pump TEE should be performed and recorded in all patients undergoing stentless tissue aortic valve replacement.