To evaluate the use of serum lactic acid values to predict flow-dependent increases in oxygen consumption (VO2) in response to increasing oxygen delivery (DO2) after blood transfusion in surgical sepsis. Prospective study. Tertiary care, trauma center. Twenty-one patients, postsurgical or posttrauma, judged septic by defined criteria. Serum lactic acid concentrations, DO2, and VO2 were measured before and after transfusion therapy. Overall, the DO2 increased from 532 +/- 146 to 634 +/- 225 (SD) mL/min.m2 (p less than .001), and the VO2 increased from 145 +/- 39 to 160 +/- 56 mL/min.m2 (p = .02). These changes occurred with an Hgb increase from 9.3 +/- 1.1 to 10.7 +/- 1.5 g/dL (p less than .001). The patients were grouped by their pretransfusion serum lactic acid values. In those patients with normal (less than 1.6 mmol/dL) serum lactic acid (n = 10), DO2 increased from 560 +/- 113 to 676 +/- 178 mL/min.m2 (p less than .02), and VO2 increased from 150 +/- 25 to 183 +/- 46 mL/min.m2 (p less than .02). However, in the increased serum lactic acid group (n = 17), VO2 was not significantly changed after transfusion (143 +/- 46 to 146 +/- 58 mL/min.m2) despite increased DO2 (515 +/- 163 to 609 +/- 251 mL/min.m2, p less than .01). Blood transfusion can be used to augment DO2 and VO2 in septic surgical patients. Increased serum lactic acid values do not predict patients who will respond. The absence of lactic acidosis should not be used in this patient population to justify withholding blood transfusions to improve flow-dependent VO2. Patients who have increased lactate concentrations may have a peripheral oxygen utilization defect that prevents improvement in VO2 with increasing DO2.