We report 11 cases of interprosthetic dislocation of the Charnley/Hastings hemiarthroplasty in a retrospective study of 350 consecutive patients treated for an acute femoral neck fracture. We found a total of 14 dislocations (4.0%), 11 were interprosthetic. The median age of the 350 patients was 79 (36-99) years and the median age of the 11 patients with an interprosthetic dislocation was 85 years (82-94). The median time from surgery to radiologically acknowledged dislocation was 18 (4-64) days. These interprosthetic dislocations may be caused by either an assembly mistake perioperatively, by maximum angulation and impingement between the components, or by trauma.