BACKGROUND One Lung Anaesthesia (OLA) is used in thoracic surgery for prevention of spillage of blood and pus into the healthy lung to facilitate surgical exposure. During OLA since the collapsed lung continues to be perfused, there will be large right to left shunt which leads to hypoxaemia. Objective-To find out the potential factors that reduce arterial oxygenation (PaO2) during OLA and to find out the possibility of predicting the PaO2 during OLA based on these potential factors. MATERIALS AND METHODS A cross-sectional study was conducted among 34 patients who came for lung or non-lung surgery in the CMC Vellore during a period of 2 years. All the ASA grade I and II patients who needed OLA were included. Preoperative PaO2, TLV PaO2, PaO2 at 10 mins. and 25 mins. after starting OLA were assessed. Alteration in PaO2 related to side, smoking and gender was measured by mean +/- SD and multiple linear regression was done to assess the independent contribution of each of 9 predictors with OLA PaO2 at 25 min. RESULTS Three factors were of significance in predicting the PaO2 which were gender, side of surgery and smoking. Women had significantly (p=0.04) higher PaO2 as compared to men. Similarly, left side had significantly (p = 0.01) higher PaO2 as compared to right side and non-smokers had significantly lower PaO2 compared to smokers (p = 0.03). A predictive equation was constructed for PaO2 on OLV at 25 mins. by using the significant predictors in this study. CONCLUSION Our study was an attempt to show that it is possible to predict preoperatively the patient who is likely to suffer from hypoxaemia during OLA. The ability to predict the subsequent arterial oxygenation allows the anaesthetists/surgeons to assess and rationalise risk/benefits regarding the use of OLA during thoracic surgery and permits more controlled intraoperative management of oxygenation.