Background The 5-year cardiac mortality rate has been reported to be more than 50% in patients with left main coronary artery (LMCA) stenosis. The purpose of this study was to visualize the LMCA using intravascular ultrasound in patients who undergo interventions of the left anterior descending coronary artery (LAD). This was done in order to see the incidence and severity of plaque formation in the LMCA in patients with severe lAD stenosis, and to address if scanning LMCA is able to rule out lAD significant stenosis or vice versa. Methods A total of 293 patients with intervention of the LAD stenosis were examined with intravascular ultrasound(IVUS). The images of 278 patients were suitable for analysis. Results Fifty-three (19%) were found to have angiographic lumen reduction in the LMCA ranging from 11% to 35% (19 ± 8%). Sixty-one patients were found to have native calcification in the LMCA. Atherosclerotic plaques in LMCA were detected in 211/278 (76%) patients, of which 164/211 (78%) were eccentric, and 51/211 (24%) had calcium deposit. The cross-sectional plaque area ranged from 1.5 mm2 to 21 mm2 (8.4 ± 4.7 mm2 ). Area of stenosis was 34 ± 14 % (8-66 % ) and diameter of stenosis was 21 ± 8% ( 7-42% ). A weak relationship concerning severity of stenosis between LAD stenosis and LMCA stenosis was found in the 59 patients with pre-interventional IVUS examinations ( r = 0.47, P ＜ 0.05 ). Condusions LMCA is frequently involved with atherosclerotic lesions in patients with severe LAD stenosis. A weak relationship does exist concerning the severity of stenosis between lAD and LMCA. However, IVUS for LMCA lesion is not able to rule out LAD stenosis.