Idiopathic protrusio acetabuli is an uncommon disease process with both primary idiopathic and secondary forms. It is important to consider all etiologic possibilities before evaluating treatment options. Diagnosis is made on the basis of an anteroposterior radiograph of the pelvis that demonstrates a center-edge angle greater than 40 degrees and medialization of the medial wall of the acetabulum past the ilioischial line. For the skeletally immature patient, triradiate fusion (occasionally combined with intertrochanteric osteotomy) provides good results. For the young adult, valgus intertrochanteric proximal femoral osteotomy is recommended. In the older adult, this procedure may provide an acceptable result if there is minimal arthritis. For patients with more advanced arthritis, total hip arthroplasty with lateralization of the cup to a normal position provides a predictable long-term solution.