To investigate the bone resorption pattern of the residual alveolar bone, the morphologic change that occurred in mandibles was analyzed with standardized lateral cephalographs of 30 edentulous patients (15 women and 15 men). The longitudinal morphologic changes were measured at the sagittal sections of the mandibular bony contour at the symphysis area on superimposed cephalographic tracings. To quantify the morphologic change, a knife-edge index (KEI) was developed as the area change divided by the height change. Geometrically, the higher value of KEI represents the greater tendency to become a narrow residual ridge. The KEI values were statistically higher in the women than in the men (p less than 0.002). In addition, the value of KEI seems to correlate with osteopenic change at the center point of the body of the second vertebra (p less than 0.01). The continuous bone resorption activity in the edentulous mandible of women seems to be emphasized at the labial and lingual surfaces of the residual alveolar bone, resulting in a knife-edge type of residual ridge.