The influence of being overweight or obese on hospital and late (1 year) mortality and reinfarction was studied in 1760 patients with acute myocardial infarction. Body mass index (BMI) was used to categorize patients as normal weight (BMI less than 25), overweight (BMI 25 to 30), and obese (BMI greater than 30). Clinical features and prognosis were compared in 658 normal weight patients, 884 overweight patients, and 218 obese patients. Complications during hospitalization and 1-year reinfarction rates following discharge were similar among the weight subsets. Hospital mortality was 13% in obese patients, similar to the 14% hospital mortality in normal weight patients, but significantly more than that in overweight patients (9%, p less than 0.05). When stratified according to age, 30% of obese patients greater than or equal to 65 years died in the hospital, compared to 13% of overweight patients (p less than 0.001), and 17% of normal weight patients (p less than 0.01). In patients less than 65 years, the obese group had a 6% mortality compared to a hospital mortality of 5% in overweight and 8% in normal weight groups (NS). In a multivariate analysis, obesity was an independent predictor of hospital death in the older, but not in the younger patient subset. One-year mortality for patients discharged from the hospital was significantly less in obese than in normal weight patients (7% vs 13%, p less than 0.05), but not different from the 11% mortality rate in overweight patients. Differences in mortality disappeared when patients were age stratified.(ABSTRACT TRUNCATED AT 250 WORDS)