This study aimed to assess the incidence, persistence, and associated mortality of severe hyperlactatemia in a large cohort of unselected critically ill patients. Also, we evaluated the association between 12 h lactate clearance, the timing of severe hyperlactatemia, and the maximum lactate levels with ICU mortality. In this retrospective, single-center study, we used data from the Multiparameter Intelligent Monitoring in Intensive Care III (MIMIC-III) database. Data extracted to screen 23,598 ICU patients for severe hyperlactatemia. A total of 23,598 critically ill patients were eligible for this study. Overall, ICU mortality in the 23,598 ICU patients was 12.1%. Of these, 760 patients had lactate concentration [Formula: see text] 10 mmol/L and ICU mortality in this group was 65%. Our findings confirm the association between hyperlactatemia and ICU mortality [odds ratio 1.42 (95% CI 1.35; 1.49; P < 0.001)]. Data for 12 h lactate clearance was available for 443 patients (276 nonsurvivable vs. 167 survival). 12 h lactate clearance yielded a high area under the curve (AUC) of 0.78, (95% CI 0.74 and 0.83). Severe hyperlactatemia is associated with extremely high ICU mortality in a heterogeneous ICU population. Lactate derived variables (the timing and persistence of severe hyperlactatemia, maximum level, and 12 h clearance) are shown to be associated with ICU mortality in patients with severe hyperlactatemia. Our results suggest that maximum lactate level and 12 h lactate clearance were clinically useful prognostic parameters for patients with severe hyperlactatemia.