Background: Studies of the association between obesity, and total mortality and cardiovascular events in patients with coronary artery disease (CAD) have shown contradictory results. We undertook a systematic review to determine the extent and nature of this association. Methods: We selected cohort studies that provided risk estimates for total mortality, with or without cardiovascular events, on the basis of bodyweight or obesity measures in patients with CAD, and with at least 6 months' follow-up. CAD was defined as history of percutaneous coronary intervention, coronary artery bypass graft, or myocardial infarction. We obtained risk estimates for five predetermined bodyweight groups: low, normal weight (reference), overweight, obese, and severely obese. Findings: We found 40 studies with 250,152 patients that had a mean follow-up of 3·8 years. Patients with a low body-mass index (BMI) (ie, <20) had an increased relative risk (RR) for total mortality (RR=1·37 [95% CI 1·32-1·43), and cardiovascular mortality (1·45 [1·16-1·81]), overweight (BMI 25-29.9) had the lowest risk for total mortality (0·87 [0·81-0·94]) and cardiovascular mortality (0·88 [0·75-1·02]) compared with those for people with a normal BMI. Obese patients (BMI 30-35) had no increased risk for total mortality (0·93 [0·85-1·03]) or cardiovascular mortality (0·97 [0·82-1·15]). Patients with severe obesity (≥35) did not have increased total mortality (1·10 [0·87-1·41]) but they had the highest risk for cardiovascular mortality (1·88 [1·05-3·34]). Interpretation: The better outcomes for cardiovascular and total mortality seen in the overweight and mildly obese groups could not be explained by adjustment for confounding factors. These findings could be explained by the lack of discriminatory power of BMI to differentiate between body fat and lean mass. © 2006 Elsevier Ltd. All rights reserved.