As part of an ongoing community-wide study examining changes over time in the incidence and survival rates of 4108 patients hospitalized with validated acute myocardial infarction (MI) in 16 hospitals in the Worcester, Massachusetts, metropolitan area during calendar years 1975, 1978, 1981, 1984, and 1986, we examined changes over time in the proportion of patients with acute MI developing atrial fibrillation (AF) and the impact of AF on in-hospital and long-term survival for up to a 10-year follow-up period. The overall percentage of patients with AF complicating acute MI was 16.0%; this proportion increased over time from 13.3% in 1975 to 14.8% in 1978, 14.9% in 1981, 20.3% in 1984, and to 17.7% in 1986. Patients with AF experienced consistently higher in-hospital case fatality rates than MI patients without AF overall (27.6% versus 16.6%), as well as during each of the 5 years under study. The independent effect of AF on in-hospital survival was not upheld, however, when a variety of potentially confounding prognostic factors were controlled for in a multivariate analysis resulting in an adjusted odds ratio (OR) of 1.18 (95% confidence interval 0.90, 1.52). Among discharged hospital patients, while the crude long-term survival rate for patients with AF was poorer than that of patients without AF for the combined as well as for individual study periods, similar to the in-hospital findings the independent effect of AF on long-term prognosis was not upheld after use of a multivariate analysis.(ABSTRACT TRUNCATED AT 250 WORDS)