SINCE FEBRUARY 1963, when the Starr-Edwards aortic ball-valve prosthesis1 was introduced into our practice, 100 consecutive operations for the replacement of the aortic valve have been accomplished without any hospital mortality. A detailed analysis of this experience may be of value in defining the reasons for this decrease in risk. Operative Series This consecutive series of 100 operations included all operations for replacement of the aortic valve except those in which another valve was also replaced. However, in seven of the operations of this series, repair of associated mitral valve disease short of valve replacement was also performed. The series of 100 operations was completed by December 1964. No patient was denied operation for aortic valve disease because of disability and failure which were too far advanced. Thus, several patients included in this experience were in advanced failure which had responded only partially to prolonged medical treatment. This is