A non-invasive method for the assessment of the degree of regurgitation in aortic and mitral regurgitation is presented. Regurgitant volume was obtained by subtracting effective stroke volume (SV eff), determined with a CO2-rebreathing method, from total stroke volume (SV tot) determined by M-mode echocardiography. Regurgitant volume and a non-invasive regurgitant index (SV tot/SV eff) were compared with semiquantitative angiography in 49 patients [mean age = 62 +/- 11 (SD) years], who on the basis of the angiography were allocated to four different groups (no, slight, moderate and severe regurgitation). Eighteen normal subjects [mean age = 26 +/- 9 (SD) years], not subjected to angiography, served as a control group and are included in the group without regurgitation. When the cube formula was used for the determination of SV tot, the average regurgitant volumes for the different groups were: 0.5 +/- 24 (SD), 15 +/- 22, 85 +/- 48 and 138 +/- 65 ml and the corresponding SV tot/SV eff ratios were: 1.0 +/- 0.3, 1.2 +/- 0.3, 2.4 +/- 0.8 and 3.8 +/- 1.5, respectively. There were significant differences (P less than 0.01) between the groups with slight, moderate and severe regurgitation respectively both for regurgitant volume and regurgitant index, but no significant difference between the group with slight regurgitation and the group without regurgitation. This study suggests that the severity of aortic and mitral regurgitation can be estimated non-invasively by the combined use of a CO2-rebreathing method and echocardiography.