Introduction: The left main coronary stem (LMS) provides blood supply to the left ventricle, and its stenosis is associated with serious clinical consequences. The accurate assessment of LMS stenosis determines appropriate treatment and long term prognosis. So far no criteria have been established to correctly estimate the magnitude of problematic lesions as indicated by quantitative angiography (QCA). Aim: An attempt to establish intracoronary ultrasound (ICUS) threshold values of significant LMS stenosis. Methods: The studied group consisted of 197 patients (mean age 69.72±8.51) who underwent percutaneous coronary intervention (PCI) of the left coronary artery. Group 1 (G1) consisted of 99 patients who had LMS diameter reduction (%DS) of less than 30%. Group 2 (G2) consisted of 77 patients with %DS between 30% and 50%, and the remaining 21 patients with %DS higher than 50% were classified as Group 3 (G3). The quantitative angiography (QCA) analysis included lumen diameter (Ldmin) which was LMS lumen diameter at the most stenotic segment as well as LMS diameter reduction (%DS). The parameters that were analysed during ICUS study included maximum plaque burden (%) (Pbmax), minimal lumen area (LAmin) and lumen stenosis (%LS) calculated according to the formula: (LAmin/LAref) x 100%. Additionally, correlations between the corresponding parameters measured using QCA and ICUS were investigated. Results: Both diagnostic techniques showed the most advanced degree of atherosclerosis in G3. All the G3 patients and 5 G2 patients had MLD values less than or equal to 2mm. In G1 LAmin values exceeded 9 mm² in all patients, whereas among G2 patients 12 (15.5%) had LAmin lower than 6 mm², 29 pts. (37.66%) within the range of 6-9 mm² and in the remaining 36 pts. (46.75%) it exceeded 9 mm². In G3 LAmin values in 17 pts. (80.95%) did not exceed 6 mm² and in the remaining 4 pts. (19.05%) were slightly higher. Lumen reduction higher than 50% was noted in all G3 patients and 3 G2 patients (in all these 3 G2 patients LAmin values were lower than 6 mm²). All G3 pts. and 3 G2 pts. with LAmin value <6mm² and %LS >50% had angina and a positive stress ECG test. All of these patients (n=24) underwent LMS stent implantation. Conclusions: 1. Minimal lumen diameter of LMS Ł2mm in quantitative angiography indicates a very high probability of significant stenosis of this vessel. 2. Ultrasound data analysis shows that besides LMS lumen area (<9 mm²) stenosis significance is determined by lumen reduction of more than 50%.