Introduction: Lactic acid in circulating blood should provide an index between balance of oxygen consumption and metabolic rate in sepsis or any state of shock. The purpose of the study was to determine the prognostic power of the lactate, the time factor of the blood lactate levels between survivors and non-survivors and the correlation between APACHE III score and blood lactate level in SIRS patients. METHOD: The study was performed on 99 patients over 16 years old who were admitted to the SICU with the criteria of SIRS. The blood lactate concentrations were assayed with arterial blood drawn in intervals ranging from 4 to 24 hours and the APACHE III scoring was done in the first 24 hours of SICU admission and daily until discharge or death for 2 weeks. The highest lactate level of the day was recorded. They were divided into two groups, survivors (n=61) and non-survivors (n=38), according to the outcome. RESULT: There were significant difference of the first day (D1) as well as peak lactate level between the survivors and the non-survivors (3.02 3.05 vs 7.41 4.78, 3.24 2.70 vs 7.82 4.88 mmol/L). Significant difference of the lactate as well as APACHE III were identified between the survivors and the non-survivors during a 14-days of observation period. Significant correlations were shown between lactate and APACHE III while the study was being conducted. The peak lactate presented superior to the D1 lactate in mortality prediction. CONCLUSION: Blood lactate concentration could be used as a prognostic index as well as APACHE III score. Serial blood lactate concentration assays are necessary to predict the outcome.