Background The prognostic value of blood lactate as a predictor of adverse outcome in the acutely ill patient is unclear. The aim of this study was to investigate if a peripheral venous lactate measurement, taken at admission, is associated with in-hospital mortality in acutely ill patients with all diagnosis. Furthermore, we wanted to investigate if the test improves a triage model in terms of predicting in-hospital mortality. Methods We retrieved a cohort of 2272 adult patients from a prospectively gathered acute admission database. We performed regression analysis to evaluate the association between the relevant covariates and the outcome measure: in-hospital mortality. Results Lactate as a continuous variable was a risk for in-hospital mortality with an odds ratio (OR) of 1.40 [95% confidence interval (CI) 1.25–1.57, P < 0.0001]. OR for in-hospital mortality increased with increasing lactate levels from 2.97 (95% CI 1.55–5.72, P < 0.001) for lactate between 2 mmol/l and 4 mmol/l, to 7.77 (95% CI 3.23–18.66, P < 0.0001) for lactate > 4 mmol/l. If the condition was non-compensated (i.e. pH < 7.35), OR for in-hospital mortality increased to 19.99 (7.26–55.06, P < 0.0001). Patient with a blood lactate at 4 mmol/l or more had a risk of in-hospital mortality equivalent to the patients in the most urgent triage category. Conclusion We found elevated admission peripheral venous lactate to be independently associated with in-hospital mortality in the acutely ill patient admitted to the emergency department. Patients with a lactate > 4 mmol/l at hospital admission should be considered triaged to the most urgent triage category.