OBJECTIVE: To study the relationship between blood lactate level and disease severity in critically ill children. METHODS: The clinical data of 232 children who were critically ill between September and December, 2010 were retrospectively studied. According to blood lactate levels within 24 hrs of admission, the 232 patients were classified into three groups: normal (n=146), high lacticemia (n=72) and lactic acidosis (n=14). The circulation functions, pediatric critical illness scores and prognosis were compared among the three groups. RESULTS: The degree of sepsis among the three groups was different (χ2 = 13.592, P<0.01). The occurrence of septic shock in the lactic acidosis group (42.9%) was significant compared with that in the normal (7.5%) and the high lacticemia groups (11.1%). The pediatric critical illness scores were different among the three groups (χ2 = 12.854, Ρ<0.05). The blood lactate level was significantly negatively correlated with the pediatric critical illness scores (r=-0.405, P=0.002). The prognosis among the three groups was also varied (χ2 = 25.599, P<0.01). The curative rate (7.1% vs 23.3%; Ρ<0.05) and the improvement rate (28.6% vs 58.2%; P<0.05) in the lactic acidosis group were significantly lower than in the normal group, and the mortality (28.6%) was significantly higher than in the normal (5.5%) and the high lacticemia groups (6.9%). CONCLUSIONS: A higher blood lactic acid level is associated with a more severe illness state and a worse prognosis.