This report presents baseline clinical and angiographic data from the Bypass Angioplasty Revascularization Investigation (BARI), a multicenter international trial assessing the relative efficacy of percutaneous transluminal coronary angioplasty (PTCA) versus coronary artery bypass graft surgery (CABG) in selected patients with multivessel coronary artery disease. PTCA is commonly performed in patients with multivessel coronary artery disease, yet its long-term efficacy in comparison to CABG is unknown. From August 1988 through August 1991, 1,829 qualifying patients with multivessel disease suitable for either procedure were randomized to PTCA or CABG; sample size estimates were based on anticipated 5-year mortality. Two registry populations were also defined for follow-up: (1) 2,013 patients eligible for randomization but not randomized; and (2) 422 patients considered by angiography as unsuitable for randomization. Patients randomized in BARI were at relatively high risk for subsequent cardiac events: 39% were ≥ 65 years old, 55% had prior myocardial infarction, 69% presented with unstable angina or non-Q wave myocardial infarction, and 43% had 3-vessel coronary artery disease. Patients randomized to PTCA and CABG were equally matched in all the important baseline variables. The randomized and the eligible but not randomized groups were similar in most respects. However, the nonrandomized group had a higher proportion with college education; fewer with a history of myocardial infarction, heart failure, diabetes, and smoking; and a somewhat better average ejection fraction. At the 3-month followup, PTCA had been performed more commonly in the nonrandomized eligible patients, especially those with 2-vessel disease. In comparison to the randomized patients, angiographically excluded patients were older; had more prior myocardial infarctions, heart failure, stable angina, complex coronary anatomy; and were less likely to undergo PTCA. The BARI randomized population is suitably composed to yield a meaningful comparison of the 5-year outcome of patients with multivessel disease eligible for either PTCA or CABG. The registry of eli gible, nonrandomized patients and the registry of angiographically excluded patients should provide important ancillary data to complement the randomized trial. © 1995, All rights reserved.