Background The aim of this article is to describe the courses of vomiting, diarrhea, fever, and clinical deterioration, in children with uncomplicated gastroenteritis at presentation. This study was performed as a 7-day prospective follow-up study in an out-of-hours primary care service. The course of vomiting, diarrhea, and fever was analyzed by generalized linear mixed modeling. Because young children ( 90%). Vomiting and fever decreased rapidly, but diarrhea decreased at a somewhat slower pace, especially among children aged 6-12 months. Children who deteriorated during follow-up had a higher frequency of vomiting at presentation and higher frequencies of vomiting and fever during follow-up. Conclusions The frequency of vomiting, not its duration, appears to be the more important predictor of deterioration. When advising parents, it is important to explain the typical symptom duration and to focus on alarm symptoms. Clinicians should be vigilant for children with higher vomiting frequencies at presentation and during follow-up because these children are more likely to deteriorate.