The echocardiographic features of a tumor in the right ventricular outflow tract that prolapsed into the pulmonary artery during systole are described. The patient was a 19 year old woman who presented clinically with bacterial endocarditis involving a mildly stenotic pulmonary valve. An echocardiogram, obtained to evaluate the pulmonary valve for bacterial vegetations, showed abnormal echoes throughout the cardiac cycle; they suggested a tumor mass in the right ventricular outflow tract in front of the pulmonary valve with possible extension into the pulmonary artery during systole. Additional tumor echoes confined to diastole were recorded in front of the aortic root and the tricuspid valve. These features were further elucidated with a computer-generated two dimensional cineechocardiogram that clearly showed a portion of the tumor mass passing beyond the position of the pulmonary valve into the main pulmonary artery in systole and returning into the right ventricular outflow in diastole. At surgery, a large myxoma was found in the right ventricular outflow tract with a polypoid extension that projected into the pulmonary artery in systole and contained a fibrinous vegetation at its tip. The left cusp of the pulmonary valve was normal, but the other two leaflets showed evidence of endocarditis. © 1977.