Intravascular ultrasound was performed in 30 patients with acute anterior myocardial infarction to evaluate occult atherosclerosis in the left main coronary artery which could not be detected by coronary angiography. The intravascular ultrasound catheter was positioned in the left main coronary artery after successful intracoronary thrombolysis, and cross-sectional coronary artery scanning was performed. Percentage area stenosis (%AS) was calculated as (plaque cross-sectional area/vessel cross-sectional area) x 100. Images of crescent-shaped eccentric plaque were obtained in all patients. The mean vessel cross-sectional area was 28.6 +/- 8.0 mm2, and mean plaque cross-sectional area was 7.7 +/- 4.2 mm2. Mean %AS was 27 +/- 15% (range from 6 to 58%). In five patients, intravascular ultrasound was repeated 3 months later, which showed increased %AS more than 15% in two patients. These findings indicate that occult atherosclerosis is present in the left main coronary artery in almost all the patients with acute anterior myocardial infarction and should be treated with caution when coronary intervention is performed.