Recent studies have suggested that both cardiac magnetic resonance (MR) and multidetector computed tomography (MDCT) can quantify aortic regurgitation (AR) by planimetry of the anatomical regurgitant orifice (ARO). However, this measurement was not compared with quantitative assessment of AR such as the effective regurgitant orifice (ERO) by proximal isosurface area (PISA) transthoracic echocardiography (TTE) or phase contrast MR. In 42 patients (34 men, age 54 +/- 11 years) we compared planimetered ARO by MDCT and MR with ERO and regurgitant volume by PISA TTE and phase contrast MR. ARO by MDCT (r = 0.87, p < 0.001) and MR (r = 0.81, p < 0.001) correlated highly with ERO by TTE. However, ARO by MDCT (27 +/- 15 mm(2), p < 0.001), but not by MR (23 +/- 13 mm(2), p = 0.58), were larger than PISA ERO (22 +/- 11 mm(2)). ARO by MDCT (r = 0.78, p < 0.001; r = 0.85, p < 0.001) and MR (r = 0.85, p < 0.001; r = 0.87 p < 0.001) correlated well with regurgitant volume by PISA and phase contrast MR. Both MDCT (small ka, Cyrillic = 0.80, p < 0.001) and MR (small ka, Cyrillic = 0.84, p < 0.001) demonstrated excellent agreement in correctly assessing the mechanisms of AR, i.e. aortic root dilatation (type I), cusp prolapse (type II) and restrictive cusp motion (type III), using surgical inspection as a reference. Measurement of ARO by both MDCT and MR allows accurate quantitative assessment of AR. Both techniques can also accurately determine the mechanism of AR.