To compare transthoracic (TTE) and transesophageal (TEE) echocardiography specifically in prosthetic valve endocarditis, 73 consecutive patients (age, 7 to 80 years) with 86 prostheses who had TTE and TEE for suspected endocarditis were analyzed retrospectively. Thirty-four patients proved to have endocarditis according to clinical criteria (pathoanatomical confirmation in 16), the remaining 39 served as controls. In the endocarditis group, a total of 38 (25 mitral, 13 aortic) prostheses were investigated. Vegetations were detected by TTE in 5 (all aortic) and TEE in 28 (20 mitral, 8 aortic) prostheses (13 vs 74%, p < 0.0OO1). An annular abscess was found on TEE in 1 mitral implant (3%). New perivalvular regurgitation was demonstrated on TTE in 7 (2 mitral, 5 aortic) and on TEE in 16 (10 mitral, 6 aortic) prostheses (18 vs 42%, p = 0.025). All in all, abnormalities suggestive of endocarditis were revealed on TTE in 10 (2 mitral, 8 aortic) and on TEE in 35 (23 mitral, 12 aortic) prostheses (sensitivity 26 and 92%, p < 0.0001). In the control group, TEE was false positive in 1 mitral prosthesis (specificity for TTE and TEE 100 and 97%, respectively; p = NS). These results indicate that TEE is markedly superior to TTE in prosthetic valve endocarditis. The diagnostic advantage is most evident in mitral prostheses. This holds for the detection of both morphologic changes and prosthetic malfunction.