OBJECTIVE Although obesity is a risk factor for coronary heart disease (CHD), it might be associated with a favourable prognosis in patients with CHD. The aim of the study was to evaluate this so called 'obesity paradox' during a follow-up period of 20 years in patients who had undergone coronary artery bypass grafting (CABG). RESULTS The study population consisted of 922 CHD patients who had undergone CABG between 1993 and 1994. Pre and perioperative data was collected from patient records and supplemented with patient questionnaires, telephone contacts and data from national archives. The 10-year postoperative prognosis of normal-weight patients (body mass index (BMI) 18.5-24.9 kg/m2) was inferior to that of overweight (BMI 25.0-29.9 kg/m2) and obese patients (BMI⩾30.0 kg/m2) and to the background population. Beyond 10 years the prognosis of obese patients deteriorated when compared with the overweight group. At the end of the 20-year follow-up, survival of the normal weight group was 0.68 (95% confidence interval (CI), 0.49-0.87; p<0.001), the overweight group 0.82 (95% CI, 0.71-0.92; p<0.001), and the obese group 0.67 (95% CI, 0.49-0.85; p<0.001), when compared with their background populations (=1.00). Obese patients developed diabetes more frequently and died more frequently of cardiovascular disease than patients in the two other study groups during the second postoperative decade ( p<0.01). CONCLUSIONS During long-term follow-up the obesity paradox seems to disappear due to progression of cardiometabolic disease in patients who have undergone CABG.