Background: Overcrowding of emergency departments (ED) during out-of-hours health care is a common problem in many Western countries. An appealing alternative for ED’s is the general practices cooperatives (GPC): regional networks of GP’s during the out-of-hours. The aim of this study is to investigate the socio-economic determinants of patients’ choice to opt for one of both health care providers. Methods: We gathered patient level data between May and December 2012 in Flanders (Belgium) from three emergency departments and three general practices cooperatives. Chi square analyses were used in order to study the bivariate relationship between opting for a health care facility (ED or GPC) and various socio-economic factors. To determine the independent association, a logistic regression analysis was conducted. Results: Self-referral to the ED is significantly associated with: being male, being younger, being less educated, having a (lower educated) partner and living in a rural area. Conclusion: The results of this study show that opting for the ED during the out-of-hours is, in general, related to a lower socio-economic status. Therefore the government should focus on patients with a lower socio-economic status when disseminating information on the organisation of health care with an emphasis on the accessibility of the primary health care during out-of-hours. To meet the needs for health care during these out-of-hours, a possible solution could be an integrated facility of GPC and ED with one access to medical care for all patients.