Aortic regurgitation (AR) and mitral regurgitation (MR) can result in serious clinical complications and death. The physiologic and clinical correlates of AR and MR in a free-living young adult population, however, have not been well defined. The prevalence and correlates of AR and MR were investigated in Coronary Artery Risk Development in Young Adults (CARDIA), a multicenter National Heart, Lung, and Blood Institute study of 4,352 men and women aged 21 to 35 years who had 2-dimensionally directed M-mode echocardiographic and spectral and color Doppler examinations. Isolated MR by color Doppler was detected in 10.4% (90.4% with trivial or mild severity). Isolated AR by color Doppler was present in 0.8% (37.7% with mild severity). Combined AR and MR occurred in 0.5%. There was no association between body mass index and the prevalence or severity of MR or AR. Left ventricular mass was greater in subjects with isolated AR (mean ± SD 172 ± 49 g) than in those with MR (155 ± 48 g) and greater in both groups than in subjects without MR and AR (148 ± 44 g). AR was associated with increased aortic root diameter, whereas subjects with isolated MR and those with AR and MR had increased left atrial dimensions and greater left ventricular internal dimensions. In conclusion, MR and AR detected by color Doppler echocardiography are relatively uncommon in a healthy young adult population, but both are associated with evidence of increased left ventricular dimensions and mass.