The present study was conducted to study the significance of lactate and arterial blood gas (ABG) as prognostic marker in patients admitted to intensive care unit receiving mechanical ventilation. This study was conducted on 50 critically ill patients between the age group of 20-60 years and 50 controls. Plasma lactate was determined among controls. Plasma lactate and ABG were determined among patients. Arterial blood gas analysis for pO2, pCO2 and pH was carried out among patients. Higher lactate, low pO2 and high pCO2 were observed among patients. Lactate was positively correlated with pCO2 and negatively with pO2 among patients. Patients developing hyperlactatemia during the course of intensive care unit treatment have poor prognosis. Lactate can be used as a prognostic tool in intensive care unit patients receiving mechanical ventilation.