In this study, we developed and internally validated a clinical model for predicting major adverse outcomes in patients undergoing percutaneous transluminal coronary angioplasty (PTCA) using a multi-institutional prospective cohort study involving all adult patients who underwent PTCA at 12 participating institutions from August 1993 to October 1995. A major adverse outcome, defined as death, renal failure, myocardial infarction, cardiac arrest, stroke, or coma, occurred in 3.3 and 3.2% of patients in the derivation and validation sets, respectively. Death occurred in 1.5% in both sets. Fourteen variables were independently correlated with major adverse outcomes. The rule, which stratifies PTCA patients into six levels of risk based on the severity score, showed excellent discrimination (receiver-operating characteristic curve area 0.82) and calibration (Hosmer-Lemeshow chi-square statistic P =.90) and performed well on internal validation. This rule allows accurate preprocedure stratification of PTCA candidates according to their risk of suffering a major adverse outcome.