A round to oval radiolucency surrounded by a thin zone of sclerosis is often identified in the proximal superior quadrant of adult femoral necks. Although usually recognized as incidental and benign, these radiolucencies may be of clinical concern in patients with hip symptoms. The true nature and genesis of these radiolucencies have not been explained. This article relates these radiolucencies to common acquired degenerative changes developing on the surface of the femoral neck in adults (the reaction area) and shows that the radiolucency represents a subcortical pit or cavity formed by herniation of soft tissue contents through defects in the surface of the reaction area. The formation of this pit and its relation to the commonly seen femoral neck radiolucency has not been previously described. The name "herniation pit" is suggested for these lesions.