BACKGROUND Frailty and multimorbidity are common among patients in geriatric rehabilitation care (GRC). Proper care of these patients involves multiple professionals which requires optimal interprofessional collaboration to provide the best possible support. Interprofessional collaboration (IPC) goes beyond multi-professional collaboration. It requires professionals to think beyond the expertise of their own discipline and work on joint outcomes in which the patient is actively involved. This study describes the development of the GRC teams of an elderly care organization towards the IPC. METHODS Mixed method pre-post study of 15 months. The interprofessional training program comprised team trainings, webinars, and online team sessions. Data was aggregated by administering the EPIS (interprofessional identity) and QuickScan (QS) (aspects of collaboration) measurements to GRC staff and by observations of the multi-professional team consultation (MPC) meetings of six GRC teams of an organization for elderly care in the Netherlands. ADL independence (Barthel Index) and number of inpatient days were analyzed before and after the project. RESULTS Pretest response was 106; posttest was 84. The EPIS shows improvement on "interprofessional belonging" (P=.001, 95%CI: .566 - 2.208), "interprofessional commitment" (P=.027, 95%CI: .117 – 1.899), and overall “interprofessional identity” (P=.013, 95%CI: .622 - 5.197). On the QS, all domains improved; "shared values" (P= .009, 95%CI: .067 - .474), "context" (P= .005, 95%CI: .077 - .435), "structure & organization" (P= .001, 95%CI: .141 - .560), “group dynamics & interaction” (P<.001, 95%CI: .179 - .576), and “entrepreneurship & management” (P= .039, 95%CI: .013 - .479). A qualitative analysis of the reflection responses and MPC observations indicate a shift from multi-professional to more IPC. The mean number of inpatient days was reduced by 11.8 (P<.001, 95%CI: -17.335 - - 6.305) days. CONCLUSIONS Within the GRC teams, there was a shift observed to more IPC and better representation of the patient's wishes and needs. ADL independence did not change, yet there was a beneficial impact on the number of inpatient days. The basis for IPC was well established, however, it remains necessary that the teams continue to develop and invest in the collaboration with each other and the patient to further improve it.