BACKGROUND AND OBJECTIVES: A significant ST segment depression is known to be an independent risk factor for acute coronary syndrome (ACS). Defining high risk groups in non ST elevation myocardial infarction (NSTEMI) is especially important due the poor long term prognosis of these patients. The purpose of this study was to determine the prognostic significance of the degree of ST depression on admission, as determined by a novel ST depression scoring system. SUBJECTS AND METHODS: 68 patients, admitted to Yonsei Cardiovascular Hospital between Jan 2001 and Aug 2002, and diagnosed with acute non ST elevation myocardial infarction were included in this study. Analysis of the initial ECG on admission was retrospectively performed. RESULTS: ST depression scores > or =1 and <1 were present in 36 (Group I) and 32 (Group II) patients, respectively. The rate of multivessel disease was significantly higher in group I than II (76.7 vs. 50%, p=0.032), the use of glycoprotein IIb/IIIa inhibitors was more frequent in group I than II (25 vs. 6.3%, p=0.041) and the left ventricular ejection fraction was significantly lower in group I than II (44.6+/-14.5 vs. 54.5+/-11.6%, p<0.05). The one-year survival rates were 68.9 and 93.7% for Groups I and II, respectively; p=0.0095), with Group I having a significantly higher early in-hospital mortality rate compared to group II.( 27.8 vs. 3.1%, p=0.0058) The event free survival rate in group I was lower than that in group II (55 vs. 90.6%, p=0.001). CONCLUSION: The ST depression score may be useful as an objective prognostic factor in acute NSTEMI, which may be especially useful for prediction of the early in hospital prognosis.