In this paper, the Gait Deviation Index (GDI) was used as a convenient method to evaluate pre-to-postoperative gait quality changes after total hip arthroplasty and identify predictors of outcome. Design: Three-dimensional gait data from a randomized clinical trial was used to determine changes in gait quality in participants walking at self-selected speed. Upon completion of the first assessment, the participants were randomly assigned to either resurfacing hip arthroplasty or conventional hip arthroplasty. The outcome was changes in overall gait quality measured with GDI during the six-month post-surgery follow-up period. Results: 38 participants with severe unilateral primary hip osteoarthritis took part in the trial. We found no difference in change scores between the two treatment groups; 1.9 [95%CI: -0.3 to 4.0] or between change scores for the non-operated and the operated limbs; 0.3 [95%CI: -2.3 to 1.7]. However, the score for the two groups (pooled data) improved after surgery by 4.4 [95%CI: 1.8 to 7.0]. The single level regression analysis identified the preoperative GDI score as a strong predictor of outcome (p < 0.001). Conclusion: Six-months after surgery, there was no additional effect of resurfacing hip arthroplasty on gait quality compared with conventional hip arthroplasty. The preoperative GDI score was a strong predictor for postoperative gait quality improvements, while there was no predictive value evident for gender, age and preoperative walking speed. The GDI increased, which indicates an overall improvement in gait quality after surgery.