The respective values of transoesophageal (TOE) and transthoracic echocardiography (TTE) in the evaluation of the mechanism and the quantification of pathological regurgitation of bioprosthetic heart valves were analysed in 23 patients (14 mitral, 9 aortic; duration of implantation 108 +/- 43.2 months). Surgical or pathological correlations were available in all cases and catheter data in 18 of the 23 patients. With regards to mitral bioprostheses, the TOE evaluations of the mechanism and site of regurgitation corresponded in all cases with the operative or pathological findings and quantification of mitral regurgitation concorded with angiography. There was an underestimation of the severity of mitral regurgitation in 30% of cases by TTE compared with angiography; prolapse was diagnosed in 7 of the 10 cases with cusp tears. It was not possible to accurately determine the intra or perivalvular site of regurgitation by TEE. With regards to aortic bioprostheses, TOE and TTE were equally useful in determining the mechanism of regurgitation, showing cusp prolapse in 6 of the 9 cases with cusp tears. However, TTE quantified regurgitation accurately in all cases with respect to angiography, whereas TOE was only contributive in 50% of cases. These results show that single plane TOE is superior to TTE in the quantification and determination of the mechanism of regurgitation in mitral bioprostheses, but that TTE remains better for the quantification of regurgitation of aortic bioprostheses.