A multi-centre double-blind randomized study is reported in which the effect on mortality of oral disopyramide (300 mg loading dose, then 100 mg qds) was compared with placebo in 1985 patients entering hospital with suspected acute myocardial infarction. Treatment was commenced with 24 hr of onset of symptoms (mean time to first dose 9 hr) and continued until discharge from hospital or 14 days, whichever came first. Nine-hundred and ninety-five patients were allocated to disopyramide and 990 to placebo. The overall mortality, calculated on an intention-to-treat basis, was 7.2% for the disopyramide and 5.6% for the placebo patients. Among those patients with proven infarction mortality was 9.5% of 687 on disopyramide and 7.4% of 716 on placebo. These differences are not statistically significant. Patients with cardiac failure or hypotension at entry did not fare worse on disopyramide, but those with a conduction defect did. Reinfarction was not significantly influenced by disopyramide. The prophylactic use of disopyramide in patients with suspected acute myocardial infarction does not reduce mortality or the incidence of early reinfarction.