Implementation of a general practitioner cooperative adjacent to the emergency department of a hospital increases the caseload for the GPC but not for the emergency department.

Background: The implementation of general practitioner cooperatives (GPC) for out-of-hours (OOH) primary care, raises the question if the location of a GPC adjacent to a hospital reduces the OOH caseload of the emergency department (ED).

Methods: Two natural experiments were used in this before-after study, the effect of the implementation of two GPCs in two different regions on the out-of-hours caseload of the local EDs was compared. One GPC was located adjacent to the ED of a general hospital, the other was not.