The reliability of non-invasive quantification of mitral regurgitation (MR) was assessed in 76 patients with angiographically demonstrated MR by comparing the results of different echo-Doppler (ED) indices with angiographic grade and MR fraction (MRF). Echo-Doppler studies were performed within 72h of right and left heart catheterizations in all patients. The following results were obtained: Transthoracic maximal relative jet area (RJA) is disappointing and differentiates only grade 1 from grade 4MR. RJA correlates better with haemodynamic data when obtained by transoesophageal echo: r = 0.78 versus r = 0.62, P less than 0.001. With the transoesophageal approach, a relative jet area greater than 50% and/or a jet width greater than 6 mm measured at the origin of the jet are strong indicators of severe MR. The ratio of mitral velocity time integral (VTI) to aortic VTI correlates with MRF: r = 0.77, P less than 0.001, n = 37. A VTI ratio greater than 1.3 identifies RF greater than 40% with a sensitivity of 80% and a specificity of 87%. A total inversion of the systolic wave of pulmonary venous flow is specific for grade 4 MR. The Doppler-derived MRF, correlates well with haemodynamic RF (r = 0.92, P less than 0.001, SEE: 7.5%).