Blood lactate levels have been shown to adequately reflect the presence and persistence of tissue hypoxia in critically ill patients; however, in some clinical syndromes (eg, sepsis or septic shock), the interpretation of increased lactate levels can be difficult. Still, in patients with critical illness, increased blood lactate levels and the persistence of hyperlactatemia have been associated with significant morbidity and mortality. In addition, the predictive power of hyperlactatemia has not changed during the past 20 years. Hyperlactatemia is not necessarily coincident with metabolic acidosis; however, the combination carries the worst prognosis, with an in-hospital mortality rate exceeding 80%. The recent development of fast and reliable point-of-care instruments and hand-held devices enables clinicians to implement frequent blood lactate measurements in diagnosis and treatment protocols. This could help to improve morbidity and mortality in critically ill patients.