We investigated the relationship between atrioventricular block and in-hospital mortality in 705 successive patients admitted with a first Q-wave myocardial infarction of the anterior or inferior wall. Second- or third-degree atrioventricular block developed in 61 (8.6 per cent) patients and was more frequent in inferior (12.4 per cent) than anterior infarctions (4.9 per cent). A multiple logistic regression identified three factors which were independently correlated with block: inferior infarction, older age and larger infarct size as determined by cardiac enzymes. Mortality was 27.9 per cent in patients with block and 9.3 per cent in those without; it was significantly higher in both anterior (47.0 per cent vs 11.8 per cent) and inferior (20.4 per cent vs 6.7 per cent) infarction groups. When age, infarct size, infarct site and block were analysed simultaneously as predictors of death, block was a significant independent prognostic factor. The relative risk of death, corrected for age and infarct size, in patients showing block was similar for anterior and inferior infarction. Analysis of deaths revealed a higher incidence of unheralded death in inferior infarcts associated with high-degree block.