Large-diameter femoral heads, which may reduce dislocation, are used without extensive in vivo research. Two studies were done to determine relative dislocation rates. Study 1 compared dislocation rates in primary total hip arthroplasties (THAs) using 28- or 38-mm heads. Study 2 evaluated dislocation rates in large-diameter (38-56 mm) metal-on-metal heads. In study 1, 136 patients with 38-mm heads via posterior approach and 160 patients with 28-mm heads via Hardinge approach had an average of 52 months of follow-up (range, 32-106 months). In study 2, 469 patients had THA with large-diameter metal-on-metal heads, with an average of 36 months of follow-up (range, 24-46 months). Harris hip scores evaluated the clinical outcomes. Study 1 had no dislocations in the 38-mm group and had 4 dislocations (2.5%) in the 28-mm group (P = .12). Study 2 had 2 dislocations (0.4%). Dislocation rates in primary THA with 38-mm bearings via posterior approach are the same as 28-mm bearings via Hardinge approach. Metal-on-metal THA with 38 to 56 mm heads is also associated with low dislocation rates (0.4%) at short-term follow-up.