Risk factors were investigated for revision for dislocation in primary total hip arthroplasties (THAs) between September 1, 1999, and December 31, 2004, as reported by the Australian Orthopaedic Association National Joint Replacement Registry. For 65992 primary THAs, the only initial diagnoses with significantly increased relative risk (RR) of revision for dislocation compared to osteoarthritis were fractured neck of femur (RR, 2.03; P < .001), rheumatoid arthritis (RR, 2.01; P < .01), and avascular necrosis (RR, 1.57; P < .05). A total of 58109 primary THAs for osteoarthritis were investigated for effect of age group, sex, and fixation method. There were 428 (0.7%) revisions for dislocation, 369 (0.8%) with a cementless acetabulum, and 59 (0.6%) with cemented acetabulum (RR, 1.59; P < .01). There is a significantly increasing risk of revision for dislocation as head size decreases (P < .001). Cementless acetabula, particularly with smaller heads, have a higher rate of revision for dislocation.