BackgroundThe collaborative care model with a care manager has previously generated beneficial results for patients with depression in terms of decreased burden of depression symptoms. A care manager function has been tested in Sweden in the PRIM-CARE RCT with successful results. The aim of the present study was to evaluate the process of implementing care managers in collaborative care for patients with depression in Swedish primary health care in the PRIM-CARE RCT.MethodsThe study followed UK Medical Research Council guidance for process evaluation.Field notes from the implementation of the PRIM - CARE RCT were used, as well as data collected from five focus group discussions with General Practitioners (n = 29) and three focus group discussions with care managers (n = 11). Data were analysed with content analysis.ResultsTraining sessions, careful preparation and extensive initial support to the care manager and staff at the Primary Care Centres were important ingredients in the implementation. The close access to facilitators, the recurrent peer support meetings, and the weekly newsletter strengthened the care manager function.ConclusionsA complex intervention adapted to the Swedish primary care context focusing on a care manager function for patients with depression could be performed through a stepwise implementation process. Financial support from the health care regions included in the study helped to reduce the impact of identified barriers. This process evaluation has revealed new and important knowledge for primary care development concerning infrastructure and organization building, knowledge sharing, and facilitating factors and barriers.Trial registrationNCT02378272 Care Manager – Coordinating Care for Person Centered Management of Depression in Primary Care (PRIM – CARE). Registered March 4 2015. Retrospectively registered.