BACKGROUND: The APACHE II scoring system has been regarded as a useful tool in the assessment of the severity of injury and prognosis for acutely ill patients. Recently, there have been many reports that multiple organ failure(MOF) score is the better predictor of the mortality of critically ill patients than any other scoring system. The purpose of this study was to compare APACHE II score and MOF score for mortality prediction in critically ill patients. METHODS: 163 critically ill patients were studied. We analyzed the correlation between the mortality rate and the scores that were produced by APACHE II and MOF scoring system within the first 24 hours in the ICU. We analyzed the correlation between each score and the number of days of ICU stay. We also calculated the mortality rate according to the number of organ failure. RESULTS: 1) The APACHE II score and MOF score of the survivors(n=129) were 9 6 and 1 1, respectively and those of nonsurvivors(n=34) were 16 7 and 5 2(mean SD), respectively. 2) The r2 was 0.62 between APACHE II score and mortality rate, and 0.77 between MOF score and mortality rate. 3) The r2 was 0.06 between APACHE II score and ICU stay, 0.01 between MOF score and ICU stay. 4) The mortality rates were 0, 2, 20, 64, 73, 75 and 100 % in 0, 1, 2, 3, 4, 5 and 6 organ failures, respectively. CONCLUSIONS: The MOF score was more sensitive predictor of the mortality of critically ill patients than the APACHE II score.