The effect of treatment with an oral screen was studied in sixteen children with incompetent lips who were compared with a control group of sixteen children who also had incompetent lips. The period of observation was 1 year. The treatment brought about retroclination of proclined maxillary incisors with a resulting decrease in overjet, in the diastema between the incisors, and in arch length. The mandibular incisors proclined somewhat. No change in general intermaxillary relation or in arch width was observed with the treatment. Neither lip morphology nor the electromyographically recorded function of the lips was affected by the treatment. The maximum force that the lips could exert to resist an external force was increased substantially by the treatment. Whether this is of value for the stability of the orthodontic treatment result is not known.