To determine the clinical significance of angiographically undetected left main coronary artery (LMCA) disease, we analyzed data from 47 patients, with a mean age of 58 years, who were examined with intravascular ultrasound (3.5 Fr, 30 MHz). For assessment of atherosclerosis, the lesion area was calculated from the ultrasound images of the formula, [(total vessel area--lumen area)/total vessel areas] x 100(%). In 37 LMCA segments of patients with significant distal coronary stenosis (> 50%), the percent intima-media area (the index) was 39 +/- 11% (mean +/- SD), significantly greater than that of 10 patients without distal disease (27 +/- 4%, P < 0.01). Among those with significant coronary stenosis, the index was markedly greater in patients with multi-vessel coronary stenosis (46 +/- 12%, n = 19) than in patients with single-vessel disease (33 +/- 9%, n = 18; P < 0.01). At three LMCA sites associated with multi-vessel disease, ultrasound analysis demonstrated disruption of the intima at the site where the guiding catheter for balloon angioplasty had been positioned. These results indicate that LMCA disease is more prominent in patients with multi-vessel distal coronary disease than in those with single vessel disease, even in the absence of angiographic stenosis. We suggest that LMCA trauma can occur where the guiding catheter for angioplasty is positioned, particularly in patients with multi-vessel distal disease.