Background - Many patients are inadvertently fasted for prolonged periods (≥ 14 hours) preoperatively due to unavoidable reasons. This study was undertaken to asses the effects of inadvertent prolonged preoperative fasting. Methods - Sixty patients of ASA class I or II scheduled for elective surgery either under general anaesthesia or subarachnoid block, who inadvertently fasted for more than fourteen hours were selected randomly and divided into two groups - Group GA (patients received general anaesthesia) or Group RA (patients received subarachnoid block). Before induction of anaesthesia, all patients were asked for the presence of various symptoms like thirst, dryness of mouth, hunger, concentrated urine etc. Samples of venous blood and urine were collected before induction of anaesthesia and at various points of time peri-operatively for analysis of blood sugar, serum electrolytes, urine for specific gravity and presence of ketone bodies. The data was analyzed using ANOVA, Chi square test and Tukey test. Results - Preoperative fasting period was statistically comparable in two groups. The most common symptoms were thirst, dry mouth and concentrated urine. Three patients developed hypoglycemia without ketonuria and 6 patients developed ketonuria at pre-induction in both groups. The mean specific gravity of urine was significantly higher during study period than their values from preanaesthetic record. There was statistically significant elevation of blood glucose (although within normal physiological limits) in both groups intraoperatively and postoperatively compared to preinduction value. At 6 hours postoperatively, 60% patients developed ketonuria in both groups. Conclusions – After prolonged preoperative fasting a significant number of patients developed ketonuria independent to technique of anaesthesia.