The severity of mitral regurgitation (MR) was determined by color Doppler echocardiography in 44 adult patients with severe symptomatic aortic stenosis before and after isolated aortic valve replacement. Preoperative MR was absent in 17, mild in 14, moderate in 11 and severe in 2 patients. Three to 388 (mean 58) days after surgery, 14 patients continued to have no MR. In the other 30 patients, MR decreased in 18 (60%), remained unchanged in 8 (27%) and increased in only 4 (13%). Furthermore, in 13 patients with significant (moderate or severe) MR, the severity decreased in 12 (92%). Thus, the severity of MR often decreases after aortic valve replacement for aortic stenosis.