Registration number: NCT05270291, https://clinicaltrials.gov/ct2/show/ Objectives: In children with acute gastroenteritis (AGE), vomiting often precedes diarrhea. To establish the diagnosis of AGE, enteropathogen detection typically relies on diarrheal stool samples. However, testing requires sufficient stool sample, which may not be easily available. Recent studies suggest that in children presenting to emergency departments with presumed AGE with isolated vomiting, an enteropathogen can be identified using rectal swabs and molecular diagnostic tests. The rate of enteropathogen detection in children with isolated vomiting due to AGE may differ in various populations. Using rectal swabs and molecular diagnostic tests, we plan to assess the proportion of children with isolated vomiting with presumed AGE in whom an enteropathogen can be identified. Methods: This will be a cohort study conducted in the emergency department(s) of one or more pediatric hospital(s) in Poland. Children younger than 5 years with the presence of ≥3 episodes of vomiting due to presumed AGE, lasting no longer than 7 days before enrollment, will be recruited. The primary outcome will be the proportion of children with isolated vomiting in whom an enteropathogen is detected. In all eligible participants, rectal swabs will be taken to perform molecular testing for detection of typical viral and bacterial enteropathogens. All children will be followed-up at 14 days after the initial contact to classify them into one of three groups (i.e., vomiting only, vomiting and diarrhea, and diarrhea only).