As part of a community-based study of patients hospitalized with acute myocardial infarction (AMI) in the Worcester, Massachusetts, metropolitan area, changes over time in the incidence rates of complete heart block (CHB) complicating AMI, and the prognostic impact of CHB on the in-hospital and long-term survival of these patients were examined. In all, 4,762 patients with validated AMI hospitalized at 16 hospitals in the Worcester metropolitan area during 1975, 1978, 1981, 1984, 1986 and 1988 constituted the study sample. The incidence rates of CHB complicating AMI remained relatively stable at 5.8% over the 13-year (1975 to 1988) period studied. The incidence rates of CHB were approximately twice as high in patients with inferior/posterior wall AMI (7.7%) as in those with anterior wall AMI (3.9%). Use of a multivariate regression analysis to control for factors affecting the incidence rates of CHB revealed that patients were at highest risk for developing CHB during the latter 2 study years (1986 and 1988). Patients with AMI developing CHB had higher in-hospital case fatality rates than did those without CHB overall, as well as during each of the 6 periods studied. The in-hospital survival associated with CHB did not improve over time. After use of a multivariate regression analysis to control for additional prognostic factors, the independent effect of CHB on in-hospital prognosis remained (adjusted risk of dying = 2.10; 95% confidence intervals = 1.37, 3.21). Patients with inferior wall AMI complicated by CHB were at significantly increased risk of dying during hospitalization compared with those without CHB (adjusted risk of dying = 2.71; 95% confidence intervals = 1.60, 4.59).(ABSTRACT TRUNCATED AT 250 WORDS)