In a double blind placebo controlled trial, 434 patients with suspected myocardial infarction were randomised to treatment with nifedipine (n = 217) or placebo (n = 217) within six hours from the onset of chest pain. During the treatment period of 48 hours, a 10 mg capsule containing nifedipine or placebo was given sublingually every four hours for 24 hours, then orally every four hours for the next 24 hours. Acute myocardial infarction was confirmed in 295 patients (146 in the nifedipine group and 149 in the placebo group). The median delay time to intervention with nifedipine in patients with acute myocardial infarction was 111 minutes. Infarct size was assessed by the estimation of release of creatine kinase isoenzyme MB and creatine kinase from blood samples taken every four hours for 48 hours. The total mean (SEM) creatine kinase MB released was 406.4 (27.2) IU/l in the nifedipine group and 345.7 (20.5) IU/l in the placebo group. Total mean (SEM) creatine kinase released was 2749.6 (165.1) IU/l in the nifedipine group and 2698.4 (145.9) IU/l in the placebo group. In hospital mortality was similar for both the nifedipine and placebo groups (6.6% and 5.8% respectively). Treatment with nifedipine in the early phase of acute myocardial infarction seems to have no effect on enzymatically measured infarct size.