Background: Lactic acidosis is still a serious concern for patients undergoing cardiac surgery. Lactic acidosis presents a challenge to be managed and controlled by health care personnel, and it is contributing to an increase in morbidity and mortality rates among patients postoperatively. Aim of study: to investigate whether lactic acidosis in patients undergoing cardiac surgery is a predictor of negative postoperative outcomes.. Research design: A descriptive exploratory research design. Setting: Cardio-thoracic intensive care unit at assuit university heart hospital. Patients: 80 Patient of cardiac surgery assigned to two groups (39 patient in non-lactic acidosis group and 41 patients in lactic acidosis group) Tools: Tool one: Open-heart surgery patient assessment tool , Tool two: Preoperative assessment tool, Tool three: Postoperative assessment tool and Tool four: Assessment the outcome of lactic acidosis among patients undergoing open -heart surgery. Results: Cardiac surgery patients with lactic acidosis have higher incidence of mortality (34.1% vs 7.7%) long length of stay in (75.6% vs 0%) with Mean± SD (10.20±4.291 vs 4.05±1.099) and high incidence of complication. Conclusions: The occurrence of lactic acidosis among cardiac surgery patients was linked with a negative outcome and a high mortality rate postoperatively. Recommendation: Reduce the occurrence of lactic acidosis intra- and postoperatively, resulting in a decrease the occurrence of complications and negative outcomes postoperatively, Further research studies are need to conduct for deepen our understanding for relationship betwwen postoperative lactic acidosis and negative outcome among cardiac surgery patients