Thirty-two children with tongue-thrusting habits and nine nonthrusters participated in this study. Twenty of the tongue-thrusting children exercised their lips twice daily by pulling on a button placed in the labial bestibule. Spring scales recorded the displacing tension on the lips; exercise continued from three to six months. Pre- and postexercise headplate tracings and dental study casts were used for measurement of alterations in tooth position. The tongue-thrusting children exhibited weaker lips initially than did the non-tongue-thrusters. The lip muaculature was strengthened by daily exercise and the improvement remained for 18 months after cessation of the exercies. No signigicant change in incisor protrusion or the open bite was observed throughout the two-year study period. We conclude that increasing the lip muscle strength in tongue-thrusters may have little effect on the dentition of children exhibiting tongue-thrusting.