At the request of the Councils of the Society of Thoracic Surgeons (STS)and the American Association of Thoracic Surgery (AATS) the Ad Hoc LiaisonCommittee for Standardizing Definitions of Prosthetic Heart Valve Morbidityreviewed the "Guidelines" published in September 1988 [3, 7, 8]. Thepurpose of the review was to update and clarify definitions within theguidelines and to consider recommendations made by other [2, 11]. Thevariety of cardiac valvular procedures has expanded since 1988; therefore,in this document the term "operated valve" indicates prosthetic andbioprosthetic heart valves of all types, operated or repaired native valvesand allograft and autograft valves. The term "operated valve" includes anycardiac valve altered by a surgeon during an operation. Much morbidity andmortality is a direct consequence of the interaction between the patientand operated valve(s), although patient variables (e.g., age, degree ofcoronary arterial disease, follow-up care, etc.) may be more responsiblefor outcomes than an operated valve. However, no set of guidelines canidentify all possible patient factors that may affect morbidity andmortality. General agreement regarding the following definitions of termsand suggestions for reporting data do not preclude more detailed analysesor constructive recommendations and investigators are encouraged toidentify relevant patient factors in addition to factors related tooperated valves. Purpose The purpose of these guidelines is to facilitatethe analysis and reporting of results of operations on diseased cardiacvalves. The definitions and recommendations that follow are guidelines, notstandards, and are designed to facilitate comparisons between theexperiences of different surgeons who treat different cohorts of patientsat different times with different techniques and materials.