A prospective study was conducted on 34 stable septic patients to determine whether mild hyperlactatemia is a marker of lactate overproduction or an indicator of lactate underutilization during sepsis. Plasma lactate clearance and lactate production were evaluated by modeling the lactate kinetic induced by an infusion of 1 mmol/kg l-lactate over 15 min. The patients were divided in two groups depending on their blood lactate: ≤ 1.5 mmol/L (n = 20, lactate = 1.2 ± 0.2 mmol/L) or ≥ 2 mmol/L (n = 10, lactate = 2.6 ± 0.6 mmol/L). The hyperlactatemic patients had a lower lactate clearance (473 ± 102 ml/kg/h) than those with normal blood lactate (1,002 ± 284 ml/kg/h, p < 0.001), whereas lactate production in the two groups was similar (1,194 ± 230 and 1,181 ± 325 μ mol/kg/h, p = 0.90). A second analysis including all the patients confirmed that the blood lactate concentration was closely linked to the reciprocal of lactate clearance (r2 = 0.73, p < 0.001) but not to lactate production (r2 = 0.03, p = 0.29). We conclude that a mild hyperlactatemia occurring in a stable septic patient is mainly due to a defect in lactate utilization.