Case: A 57-year-old man with a known case of stiff person syndrome (SPS) presented with an insidious-onset 3-month history of right groin pain and inability to bearweight on the right lower extremity. Radiographs demonstrated a displaced right neck of femur fracture. Pelvic magnetic resonance imaging was negative for any infective or neoplastic pathology. Right primary cementless total hip replacement was performed. Significant improvement in function was noted after surgery. Conclusions: Femur neck fracture can result from severe muscle spasms and stiffness. Primary total hip replacement is a difficult but reasonable operative procedure for SPS with neck of femur fracture. The success of the procedure also depends on ideal medical control of muscle spasms and rigidity.