We read with interest the article by Nichol and colleagues [1] in a recent issue of Critical Care. Their study of more than 7,000 medical and surgical patients supports the claim that an admission plasma lactate level in the upper normal range is associated with increased mortality. However, we are concerned about the lack of data regarding patient baseline characteristics. Specifically, although 3,166 and 1,614 patients had diagnoses of 'surgery' and 'cardiac/vascular', respectively, it is likely, but not stated, that numerous patients were admitted following cardiac surgery. This is of great importance as changes in lactate in this patient group are not homogenous in nature. In post-cardiac surgery patients, early hyperlactatemia and late hyperlactatemia (LHL) differ in both risk profile and physiological rationale. Early hyperlactatemia (on intensive care unit arrival) is associated with adverse outcome. This association is not seen in the 10% to 20% of patients who develop LHL (pooled odds ratio [OR] of death with LHL in two published trials 1.39, 95% confidence interval [CI] 0.28 to 7.04) [2-4]. Moreover, in our recent single-center review of prospectively collated data from 529 post-cardiac surgical patients in a tertiary Australian cardiac surgical intensive care unit, 25% developed LHL (>2.53 mmol/L). When compared with patients with a normal lactate profile, patients with LHL showed no increase in hospital mortality (OR 0.57, 95% CI 0.07 to 5.05) (unpublished data). Therefore, we believe that inclusion of such patients in studies of lactate in critical illness should be avoided and, as in the study by Nichol and colleagues, may actually weaken any association demonstrated between lactate levels and hospital mortality.