OBJECTIVES: Belgium is one of the few countries to implement rotavirus vaccination within its universal paediatric immunisation programme. This study was designed to measure the impact of rotavirus vaccination on acute gastroenteritis (AGE) related emergency room (ER) visits in children aged ≤5 years in Belgium. METHODS: A retrospective observational study was performed across 11 Belgian participating centres. Each centre provided an anonymised list of all ER visits of children aged ≤5 years during the periods June 2004–May 2006 (pre-vaccine period) and June 2007–May 2009 (post-vaccine). A sample of 7620 ER visits (3699 pre-vaccine; 3921 post-vaccine) was randomly selected from the 180,253 ER visits reported. Patient demographics and information on the reason for ER visit were collected from all randomly selected visits. If the visit was AGE-driven, additional data were collected on vaccination status, symptoms, tests performed, treatment, and visit outcome. RESULTS: The proportion of AGE-driven ER visits was 11% in the 2-year pre-vaccine period (N AGE-driven visits / Total N ER visits = 412/3,699). After insignificant change to 10.5% during the 1st year after vaccine introduction (N=208/1,977), this proportion significantly declined to 8.2% (26.6% reduction; p-value <0.001) in the 2nd year (N=159/1,944). A strong seasonal effect exists in the proportion of AGE-driven ER visits on the total number of ER visits, peaking in February and March. The number of patients who needed to be hospitalised because of AGE declined over time (from a yearly average of 94 pre-vaccine to 62 and then 44 in the 2 consecutive years post vaccine introduction). Oral and IV rehydration are the most commonly used treatments. Their use also declined after vaccine introduction. CONCLUSIONS: A significant decline in AGE-driven ER visits and subsequent hospitalizations was observed in Belgium after introduction of a universal mass vaccination program against rotavirus. AGE cases appeared less severe.