In a multicenter double-blind study, 227 patients with suspected acute myocardial infarction (AMI) were randomized within 12 hr from onset of symptoms to treatment with nifedipine (112 patients) or placebo (115 patients). AMI was confirmed in 74 patients on nifedipine and in 83 on placebo. Patients with AMI received nifedipine 5.5 +/- 2.9 hr (mean +/- SD) after onset of symptoms. Infarct size was assessed by the release of creatine kinase isoenzyme MB (CK-MB). Infarct size index (CK-MB geq/m2) was 25 +/- 16 (n = 71) in the nifedipine group and 23 +/- 13 (n = 77) in the placebo group (NS). After the first 10 mg of nifedipine systolic blood pressure fell from 147 +/- 30 to 135 +/- 28 mm Hg (p less than .01) and heart rate rose from 75 +/- 18 to 79 +/- 19 beats/min (p less than .01). No change was observed after the first placebo dose. The treatment was continued for 6 weeks. Over this period there were 10 deaths in each group. Early treatment with nifedipine in patients with AMI does not seem to reduce infarct size as determined by enzyme level.