Premature closure of an atrioventricular valve was observed in 80 of a group of 519 subjects catheterized for aortic insufficiency (AI) or mitral insufficiency (MI), or both. Sole premature mitral closure (PMC) was present in nine subjects, sole premature tricuspid closure (PTC) in 40, and combined PMC and PTC in 31. Clinically PMC was associated with a first sound that was soft or absent in 75% and an atrial contraction sound in 50% of the subjects with dominant aortic insufficiency. PMC or PTC, or both, were verified at surgery in three subjects and were produced experimentally in intact dogs by acute AI. The presence of PMC and PTC appears related to the severity and chronicity of valvular disease.