In patients undergoing coronary artery bypass surgery while under 40 years of age, prevalence of risk factors for coronary artery disease and prognostic factors for mortality were assessed. Ninety-four patients of such age were operated from 1979 until 1989. Eighty-two patients were male, 12 female. Fifty-eight patients (62%) had a history of myocardial infarction. Patients were predominantly in New York Heart Association class II or III (69%). In this study prevalence of risk factors for coronary artery disease was different compared to studies of patients undergoing coronary artery bypass grafting over 40 years of age. Smoking (83%), hypercholesterolaemia (79%), and positive family history for coronary heart disease (66%) were more prevalent in this study compared to other studies concerning patients older than 40 years of age whereas the occurrence of hypertension (25%) and diabetes (6%) was lower. Six patients (6%) had one-vessel disease, 15 patients (17%) had two-vessel disease and 73 patients (78%) showed triple-vessel disease. Similar studies of patients older than 40 years of age showed a lower prevalence of triple vessel disease than in this study. Hospital mortality amounted to 3 patients (3%). Five-year-survival rate was 93%. Multivariate survival analysis revealed left main stenosis and concomitant operations as significant prognostic factors of survival. Number of vessels diseased, sex, priority of operation, reoperation, or impaired left-ventricular function were not prognostic factors for mortality in this study.