AIMS: To evaluate the importance of chronic obstructive pulmonary disease for prognosis and diagnosis of congestive heart failure in patients with acute myocardial infarction. METHOD AND RESULTS: Prospective registration of 6669 consecutive patients admitted with infarction and screened for a randomised controlled trial. A history of COPD was present in 765 (11.5%) patients. Thirty-day and 5-year survival in patients with chronic obstructive pulmonary disease was 86.3 and 42.9%. In patients without pulmonary disease the figures were 87.7 and 57.5%, respectively, giving a relative risk of 1.49 (1.35-1.65). In multivariate analysis the relative risk was 1.15 (1.04-1.28). The prevalence of congestive heart failure was 65.9% in patients with chronic obstructive pulmonary disease and 52.0% in patients without. This difference was most distinct in patients with normal or only slightly decreased left ventricular systolic function. In patients without congestive heart failure, chronic obstructive pulmonary disease was of prognostic importance [RR=1.44 (1.17-1.78)], but not in patients with congestive heart failure [RR=1.09 (0.96-1.23)]. CONCLUSION: Chronic obstructive pulmonary disease is a predictor of long-term mortality in patients with acute myocardial infarction without congestive heart failure, but is also a confounding factor for the diagnosis of congestive heart failure.