Introduction: Serum lactate elevation has been used as a marker of tissue hypoxia in the setting of perioperative monitoring and critical care. It has also been correlated with perioperative morbidity and mortality. Cytoreductive surgery (CRS) & hyperthermic intraperitoneal chemotherapy (HIPEC) for the treatment of peritoneal carcinomatosis is a major abdominal operation, requiring meticulous perioperative care. The aim of this study is to assess the clinical use of lactate measurements after CRS & HIPEC and its prognostic value in terms of immediate postoperative morbidity and mortality, as well as long term survival. Material & Methods: Of 140 patients diagnosed with peritoneal carcinomatosis who underwent cytoreductive surgery and HIPEC were prospectively studied. Serum lactate was measured by air blood gases analyser intraoperatively (just before the administration of HIPEC) and then daily till the fifth postoperative day. Postoperative complications were recorded and divided into two groups according to Clavien Dindo classification. Results: Intraoperative lactate measurements have not been associated with postoperative morbidity and mortality. On the other hand, lactate measurements on postoperative days 3 and 4 are of clinical significance. Specifically, an increase of 1 mmol/L of the average lactate value of days 3 and 4 raises the risk of a minor complication (Grades I to IIIa) by 1.9, the risk of a major complication (Grades IIIb to V) by 10.9 and the risk of mortality by 32.1%. Conclusions: The average of day 3 and 4 postoperative day lactate level is an independent predictor of morbidity and mortality in patients undergoing CRS and HIPEC.