During the acute phase of vomiting, even a small amount of water may not be tolerated by mouth. Early refeeding may cause re-vomiting in patients, whereas late refeeding may result in dehydration and hypoglycemia. Nil per os (NPO) may be generally recommended by primary physicians, but the appropriate NPO duration for these patients is still unclear.The study aimed to identify the ideal NPO duration for patients with acute vomiting.We prospectively recruited patients with vomiting who underwent NPO management and were administered antiemetic agents in the emergency department (ED) and the pediatric ED. The demographics, final diagnosis, clinical manifestations, medical management, NPO duration, and laboratory data were collected and analyzed to identify the ideal NPO durationA total of 304 patients with vomiting who were admitted in the ED were enrolled. The major diagnosis was acute gastroenteritis (AGE) (82.9%), followed by acute gastritis and colitis. Most patients were younger than 6 years (43.8%). Apart from abdominal pain and vomiting, nausea was the most common symptom (93.1%). NPO duration of 4 to 6 hours had the lowest rate of refeeding failure (3.7%) compared to the other NPO durations.For patients with acute vomiting who are admitted to the ED, NPO duration of 4 to 6 hours may be necessary and should be recommended by primary ED physicians.