Blood lactate elevation in critically ill patients commonly is taken as a sign of impaired tissue perfusion. Simultaneous elevation of lactate to pyruvate ratio (L/P ratio) may be helpful in discriminating between different mechanisms of hyperlactatemia and thus in determining the relevance of the finding. We studied prospectively the prevalence and the time pattern of hyperlactatemia and simultaneous L/P ratio elevation in 98 consecutive emergency admission patients in a 23-bed surgical-medical University Hospital intensive care unit. Blood lactate, L/P ratio, and blood gases were measured at 2-h intervals during the initial 24 h of intensive care unit admission. Hyperlactatemia (blood lactate over 2 mmol/L) was found in 48 (49%) patients, and the median peak value of the non-survivors was higher than that of the survivors [5.3 (interquartile range 1.9-7.5) vs. 1.9 (1.3-2.9) mmol/L, respectively, p = 0.003]. Hyperlactatemia at admission (n = 31) was associated with a higher hospital mortality than hyperlactatemia developing later (n = 17) (29.0% vs. 5.9%, P = 0.003). Sustained admission hyperlactatemia (>6 h) was associated with higher mortality than short-lasting hyperlactatemia (36.8% vs. 0%, P = 0.008). Simultaneously elevated L/P ratio (L/P ratio > 18; n = 16) was associated with higher mortality than hyperlactatemia with normal L/P ratio (n = 32; 37.5% vs. 12.5%, respectively, P = 0.03) and was found mainly in patients who had severe circulatory failure. The hyperlactatemia of patients with sepsis was not associated with L/P ratio elevation. We conclude that hyperlactatemia is common in emergency admission patients. Hyperlactatemia with L/P ratio elevation and lactic acidosis is likely to be associated with inadequate tissue perfusion. Hyperlactatemia persisting more than 6 h and simultaneous elevation of L/P ratio are associated with increased mortality.