Preoperative psychological distress has been reported to predict poor outcome and patient dissatisfaction after total hip arthroplasty (THA). The purpose of this study was to investigate if pre-operative psychological distress was associated with adverse functional outcome after primary THR. We analysed the database of a prospective multi-centre study undertaken between January 1999 and January 2002. We recorded the Oxford Hip Score (OHS) and SF36 score preoperatively and up to five years after surgery for 1055 patients. We dichotomised the patients into the mentally distressed (Mental Health Scale score - MHS =56) and the not mentally distressed (MHS >56) groups based on their pre-operative MHS of the SF36. 762 (72.22%). Patients (595 not distressed and 167 distressed) were followed up at 5 years. Both pre and post-operative OHS and SF-36 scores were significantly worse in the distressed group (both p<0.001). However, both groups experienced statistically significant improvement in OHS and MHS, which was maximal at 1 year after surgery and was maintained over the follow up (p=0.00). There was a substantial improvement in mental distress in patients who reported mental distress prior to surgery. The results suggest that pre-operative psychological distress did not adversely compromise functional outcome gain after THA. Despite having worse absolute values both pre and post operatively, patients with mental distress did not have any less functional gain from THA as measured by improvement in OHS.