More than half of sciatic nerve palsies after primary total hip arthroplasties are unexplained. In 2 such cases, magnetic resonance imaging localized compressive injury between the ischial tuberosity and femoral insertion of the gluteus maximus. After these cases, we hypothesized that during limb positioning, the sciatic nerve is compressed by the gluteus maximus tendon. We present the magnetic resonance imaging findings from these 2 cases and compare the number of sciatic nerve palsies in patients with release of the gluteus maximus tendon to patients without release during primary total hip arthroplasty. There were no cases of sciatic nerve palsy when the gluteus maximus was released, compared with 3 cases in the control group.