In order to determine the effect of obesity on the results of coronary artery bypass graft (CABG) surgery, we compared 250 obese patients undergoing CABG procedures between 1984 and 1987 with 250 age- and sex-matched controls of normal body mass index (BMI) undergoing CABG in the same period. The obese group had a greater incidence of diabetes mellitus (p less than 0.02), hypertension (p less than 0.05), hyperlipidaemia (p less than 0.05), and left main stem coronary artery disease (p less than 0.001). No differences were identified in the surgery performed, but obesity was associated with prolonged total bypass time (p less than 0.05). Operative mortality was 0.8% in both groups. Multivariate analysis demonstrated obesity to be an independent risk factor for perioperative morbidity (p less than 0.05). Univariate: respiratory (p less than 0.01); leg wound (p less than 0.001); myocardial infarction (p less than 0.02); arrhythmias (p less than 0.02); sternal dehiscence (p less than 0.02). At a mean follow-up time of 36.9 months obese patients exhibited a greater incidence of significant recurrent angina (p less than 0.01), which was associated with further weight gain (mean 12.2 kg; linear correlation: p less than 0.001, r = 0.891). Although in CABG surgery operative mortality is not increased in obese patients, aggressive pre- and postoperative weight control is indicated to reduce both perioperative morbidity and the incidence of recurrent angina.