Changes in lip morphology and area, measured in two dimensions from standardized lateral head films, were assessed in a series of twenty adults at three times: pre-surgically, 8--14 months post-surgically, and a long-term follow-up at 5--7 years. All individuals received the same, single surgical procedure (Obwegeser sagittal split for correcting mandibular prognathism. Upper and lower lip changes were quantified as millimetres displacement of the lip centroid vertically and horizontally, plus changes in cross-sectional area. Direction and amount of change, its dependency on the amount and kind of surgically induced symphyseal changes, and the intercorrelations among lips and among lip and symphysis variables were statistically evaluated, both univariately and multivariately. Three measures are made of symphysis change: horizontal and vertical repositioning and amount and direction of rotation. Horizontal repositioning primarily affected the lengthening and areal increase of the upper lip. Vertical repositioning had its major influence in the height and cross-sectional area of the lower lip: a superior shift of the mandible made the lower lip shorter, more protrusive and smaller in area; an inferior shift produced an increase in lower lip height with increased area. The third variable, symphyseal rotation, had its greatest influence on the labial-lingual shift of the upper lip's centroid. The long-term follow-up showed little change from the 1 year post-operative conditions; equilibrium of the soft tissue components was then achieved fairly soon after surgery.