Hip dislocation is a major short-term complication after total hip arthroplasty (THA). One factor thought to reduce the risk for dislocation is head size. We constructed subject-specific computer models to study the effect of head size on risk for postoperative dislocation. Femoral and acetabular geometry was constructed after segmenting CT scans of nine hips. CAD models of THA components with four head diameters (28, 32, 36, and 44 mm) were virtually implanted. Hip capsular ligaments were simulated using rigid-body ellipsoids connected by non-linear springs. Posterior dislocation was simulated during a rise from a low chair; anterior dislocation was simulated during a pivot activity. Intraoperative stability tests were simulated for anterior or posterior dislocation. While rising from a low chair (posterior dislocation) and during the pivot activity (anterior dislocation), increasing head size significantly increased hip flexion angle at dislocation and generated higher dislocation moments. Larger heads reduced the risk for dislocation. Intraoperative stability tests detected the relative increased resistance to dislocation despite differences in the absolute magnitude of moments. This model can be useful preclinical tool for assessing design changes, the effect of component placement, and the activity-based risk for dislocation. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:1525–1531, 2014.