Background : The APACHE II score system that evaluates prognosis has been widely applied for ICU patients. As the advent of APACHE III approaches, a comparison of effectiveness between APACHE II and APACHE III is demanded. The purpose of this study is to evaluate the relationships between APACHE II score and mortality rates, and between APACHE III scores and mortality rates in intensive care unit patients. Methods : 289 adult ICU patients participated in this study. Their mortality rates and scores on APACHE II and APACHE III were calculated. The scores of the APACHE II and APACHE III systems were also compared between survivor and non-survivor groups. Results : APACHE II scores at admission and discharge were 9±5, 6±4 in the survivor group and 20±9, 28±11 in the non-survivor group. APACHE III scores at admission and discharge were 29±19, 20±14 in the survivor group 75±37, 111±41 in the non-survivor group. The odds ratio between the mortality rate and the APACHE II score was EXP (0.2167) and the odds ratio between mortality rate and APACHE III score was EXP (0.0621). The determinant coefficient (R2) was 0.73 between the APACHE II and APACHE III scores. Conclusions : The results showed that both the APACHE II and APACHE III score systems are effective in predicting mortality rates in intensive care unit patients. (Korean J Anesthesiol 1999; 37: 814∼818)