The Collaborative Study in Coronary Artery Surgery (CASS) is a large multi-institutional study of the medical and surgical treatment of coronary artery disease (CAD). Fifteen cooperating institutes have carried out isolated coronary artery bypass grafting (CABG) on 6,176 patients from August, 1975, through December, 1978. The operative mortality (OM) was 2.3%. In an effort to better understand the clinical and angiographic characteristics predictive of OM, we have done a multivariate discriminant analysis of variables associated with OM. Numerous clinical and angiographic variables were selected from the CASS data file and evaluated in a univariate manner for their relationship to OM. Twenty of these variables were then selected for multivariate discriminant analysis. Clinical variables of most predictive value were age, female sex, increased heart size, and congestive heart failure (CHF). Angiographic variables of importance included left ventricular wall motion abnormalities, and left main coronary disease (LMCD). The priority of operation (elective, urgent, or emergent) was also associated with OM. Six variables that contained the most predictive information were selected by discriminant analysis for a group of 6,176 patients who had isolated bypass operations. In descending order of importance they were age, left main coronary artery stenosis greater than or equal to 90%, female sex, left ventricular wall motion score, left ventricular end-diastolic pressure (LVEDP), and râles. Five other groups or subgroups of patients were also analyzed in a similar manner. There is a strong association of OM with advanced age, female sex, and variables associated with left ventricular dysfunction. The risk of OM for an individual patient may be estimated with the use of these clinical and angiographic characteristics.