Changes in lip curvature resulting from treatment have been largely ignored in orthodontic literature. The focus instead has been primarily directed at retraction of the vermilion border and changes in the nasolabial angle. This study, therefore, was designed to retrospectively analyze changes in the upper and lower lip curves associated with growth and treatment. The lateral cephalometric records of 137 female orthodontic patients were digitized. Sixty-two were treated with premolar extractions and 75 without extractions. The overall extraction group was further divided into subgroups on the basis of the chosen extraction sequence, which included extraction of 4/4, 4/5, or 5/5. Statistical analysis revealed no significant differences in changes in lip curve depth between the two overall samples, relative to either of the two reference lines. This would suggest that an appropriately selected plan, whether extraction or nonextraction, should allow treatment to be carried out without negative effects on the curvature of the lips. Calculation of correlation coefficients and regression analysis suggested that the inherent properties and morphology of the soft tissues themselves are probably the greatest determinants of lip curve behavior with treatment. The midface soft tissues appear to be less dependent on changes in the underlying hard tissues than do the lower face soft tissues. Pretreatment upper and lower incisor positions and angulations and the underlying vertical facial dimension appear to play more significant roles in the behavior of the lower lip than the upper lip.