The anion gap is commonly used as a screening test for the presence of lactic acidosis. Analysis of the distribution of anion gaps for 56 adult surgical ICU patients with peak blood lactate levels >2.5 mmol/L showed the anion gap to be an insensitive screen for elevated lactate in a critically ill, hospitalized population. All patients (11/11) with a peak lactate >10 mmol/L had an anion gap >16 mmol/L; however, 50% (6/12) of patients with lactates between 5.0 and 9.9 mmol/L and 79% (26/33) of those with lactates between 2.5 and 4.9 mmol/L had anion gaps < 16 mmol/L. Hyperlactatemia was associated with considerable mortality at all levels: 100% among patients with lactate levels >10 mmol/L, 75% between 5.0 and 9.9 mmol/L, and 36.4% between 2.5 and 4.9 mmol/L. Acidosis (pH < 7.30) did not significantly alter mortality by lactate level. The observation that, for 57% of patients in this study, an elevated lactate level was not accompanied by an elevated anion gap suggests that hyperlactatemia should be included in the differential diagnosis of nonanion gap acidosis.