Objective:To evaluate the prognosis of senile critical patients over 75 years old by continuous monitoring blood lactic acid and D-dimer. Methods:One hundred and twenty-six critical patients over 75 years old were included in this research,and divided into two groups:improved surviving group and poor prognostic death group. Blood lactic acid and D-dimer were continuously monitored for the first week at admission into ICU. Results:Of 55 cases in the surviving group,APACHE II score was 18.8 ± 4.5 at admission in ICU,meanwhile lactic acid was (1.7 ± 1.3) mmol/L,and D-dimer was (2.03 ± 1.55) mg/L. Of 71 cases in the death group,APACHEⅡ score at admission was 25.3 ± 6.8,lactic acid was (2.4 ± 1.6) mmol/L,and D-dimer was (2.36 ± 1.07) mg/L. At admission,only APACHEⅡ score in the death group was significantly higher than that in the surviving group. After a week stay in ICU,lactic acid was (1.2 ± 0.6) and (5.6 ± 4.4) mmol/L,D-dimer was (2.05 ± 0.81) mg/L and (3.19 ± 1.68) mg/L in the surviving and the death group, respectively. Both indicators were significantly higher in the death group. Conclusion:Continuously monitored lactic acid and D-dimer were important indicators to evaluate the clinical prognosis of senile patients. The prognosis would usually be better if both lactic acid and D-dimer keep decreasing during the first week at admission in ICU.