Purpose: The purpose of this study was to investigate the potential additional value of cardiac magnetic resonance (CMR) in the assessment of left ventricular (LV) dilatation and dysfunction by comparison to standard echocardiography in patients with chronic left-sided valvular regurgitation.Materials and methods: We prospectively enrolled patients with chronic severe mitral regurgitation (MR) or aortic regurgitation (AR). They underwent standard echocardiography and CMR using aortic flow and LV-function sequences. LV dilatation or dysfunction was assessed with each technique, based on thresholds used for surgery indication. Reference regurgitation severity was defined following previously reported CMR-based regurgitant volume thresholds.Results: A total of 71 patients with chronic severe MR (n= 44) or severe AR (n= 27) were prospectively included. There were 60 men and 11 women with a mean age of 61 + 14 (SD) years (range: 18-83 years). CMR-based regurgitation severity was significantly greater in the LV dysfunction group when assessed with CMR (MR, P = 0.011; AR, P= 0.006) whereas it was not different when LV dysfunction was assessed using standard echocardiography. Among standard echocardiography and CMR volumetric indices, CMR-derived end-diastolic volume showed the best ability to predict regurgitation severity (area under the curve [AUC] = 0.78 for MR; AUC = 0.91 for AR). Diagnostic thresholds identified on receiver operating characteristics-curve analysis were lower than those of current European recommendations and closer to North-American guidelines.Conclusion: CMR assessment of LV end-diastolic volume in chronic severe left-sided regurgitations is more reliably associated with CMR-based regurgitant volume by comparison with standard echocardiography diameter. CMR may provide useful evaluation before surgery decision for severe asymptomatic regurgitations. (C) 2020 Societe francaise de radiologie.