Background: Gastric juice volume and acidity are influenced by food, drugs, and patient factors such as age, sex, weight, and fasting time. But almost studies were performed without controlling of these confounding variables. This study was designed to determine the change of gastric juice volume and acidity in relation to the fasting time within a constant food intake. Methods: Fifty-seven healthy patient randomly allocated to one of two groups. Group I was instructed to ingest 200 ml of milk up to 8 hours before anesthesia and Group II was instructed to drink 200 ml of water up to 1~2 hours before anesthesia. After the endotracheal intubation the volume and acidity of gastric juice were measured. Relationships between gastric volume or acidity and fasting time were analysed using correlation analysis. The data were analyzed using Wilcoxon rank sum test, comparing Group I with Group II. Results: There is no significant relationship between gastric juice volume or acidity and the fasting time within the same food intake. Gastric juice volume and acidity of the patients who ingested milk at 8~9 hours before anesthesia were similar to that of those patients who drank water at 1~2 hours before anesthesia. Conclusion: It is necessary to individualize the fasting time of the patients and it seems that water ingestion up to 1~2 hours before anesthesia may be safe.