Aim To investigate the effect of internal fixation versus hip arthroplasty on physical function and pain in elderly patients. Methods This is a substudy of an on-going nationwide multicenter registry-based RCT named HipSTHeR- (Hip Screws or Total Hip replacement or undisplaced femoral neck fracture in elderly patients) \[21\]. The aim is to recruit 340 patients with undisplaced or minimally displaced femoral neck fracture, Garden 1 or 2, are randomized to surgery either with internal screw fixation or hip arthroplasty. This substudy aims to investigate the potential differences between internal fixation versus hip arthroplasty regarding physical function and pain 4- and 12- months after surgery. Those included in the main study will be contacted by letter a few weeks after the surgery to be invited to participate in a further study. The assessor will then contact the person by telephone to answer further questions and to obtain informed consent. The follow-ups will be conducted over telephone and mail at 4 and 12 months. During the follow-up the participants will answer questions about their functional level. Information on randomisation and fracture data will be acquired from the Swedish Fracture Register and treatment data from the Swedish Hip Arthroplasty Register. Primary outcome The New Mobility Score will be used as the primary outcome with follow-up at 4- and 12 months. Secondary outcome WOMAC will be used as an additional hip specific patient reported outcome. Activities of daily living will be assessed with Katz ADL index to evaluate the patients' performance and the need of assistance in ADL. Patients' cognitive status will be assessed ALFI-MMSE, adapted from Adult Lifestyles and Function Interview (ALFI-MMSE). The Geriatric Depression Scale (GDS-15), which are developed to identify depressive symptoms. The Philadelphia Geriatric Center Morale Scale (PGCMS) and have been used in several studies including frail people living in residential care facilities and can be used among patients with cognitive impairment.