OBJECTIVE: To compare two solutions for fluid resuscitation in post-coronary artery bypass grafting (CABG) surgery patients: Ringer's lactate (RL) versus a new solution containing half-molar sodium-lactate (HL). DESIGN: Prospective randomized open label study. SETTING: The first 12 h post-CABG surgery in an intensive care unit (ICU). PATIENTS: There were 230 patients enrolled in the study: 208 were analyzed, with 109 from the HL group and 99 from the RL group. INTERVENTIONS: Patients received over the first 12 h post-CABG 10 ml kg BW(-1) HL solution in the HL group versus 30 ml kg BW(-1) of RL solution in the RL group. MEASUREMENTS AND RESULTS: Hemodynamic status, body fluid balance and inotrope utilization were compared in the two groups. Post-operative cardiac index increase was significantly higher in HL than in RL (P = 0.02), while mean arterial pressure and other hemodynamic parameters were comparable together with urinary output, indicating similar tissue perfusion in both the groups despite a much lower fluid infusion in the HL group. Therefore, a significant negative fluid balance was achieved in the HL but not in the RL group (-790 +/- 71 vs. +43 +/- 115 mL 12 h(-1), P < 0.0001 for HL and RL, respectively). None of the enrolled patients exhibited side effects related to the treatment. CONCLUSION: Half-molar lactate solution is effective for fluid resuscitation in post-CABG patients. Compared to Ringer's Lactate, its use results in a significantly higher cardiac index with less volume being infused, resulting in a very negative post-operative body fluid balance.