BACKGROUND: Total hip replacement provides pain relief and improves physical function and quality of life in patients with end-stage hip osteoarthritis. The incidence of hip replacement operations is expected to increase due to the growing elderly population. Overall, the posterior approach and lateral approach are the two most commonly used approaches for hip replacement operations. The posterior approach is associated with an increased risk of revision due to dislocations, and some studies have shown that the lateral approach is associated with reduced patient-reported outcomes, including physical function and pain; however, this has not been investigated in a randomised controlled trial with a twelve-month follow-up. We hypothesized that the lateral approach has an inferior outcome in patient-reported outcome compared with the posterior approach after one year. METHODS/DESIGN: The trial is a prospective, double blinded, parallel-group controlled trial with balanced randomisation [1: 1]. Patients with hip osteoarthritis scheduled for hip replacement surgery, aged 45-70 years, will be consecutively recruited and randomised into two groups. Group A will receive hip replacement using the posterior approach, and Group B will receive hip replacement using the lateral approach. The primary end-point for assessing the outcome of the two interventions will be twelve months after surgery. Follow-up will also be performed after three and six months. The primary outcome is Hip Disability and Osteoarthritis Outcome Score, subscale of "Physical function Short form" (HOOS-PS) Secondary outcome measures include two other subscales of HOOS ("Pain" and "Hip related Quality of Life"), physical activity level (UCLA activity score), limping (HHS) and general health status (EQ-5D-3L). Explorative outcomes include physical function test, 3D-gait-analysis and muscle strength. DISCUSSION: To our knowledge, this is the first randomised controlled trial comparing the posterior approach with the lateral approach with patient reported outcome as the primary outcome and with a twelve-month follow-up. TRIAL REGISTRATION: Clinicaltrial.gov: NCT01616667.