AIM: This retrospective study was designed to compare the responses of the upper and lower lip curves in young growing males, to orthodontic treatment involving either four premolar extractions or the holding of the E-spaces without premolar extractions. METHOD: Pre- and post-treatment lateral cephalograms of 67 premolar extraction and 30 nonextraction patients were assessed. The results were subjected to analysis of variance to search for statistically significant differences amongst the pretreatment variables and changes in depths of lip curves. Pearson's correlation coefficients were also calculated and stepwise regression analysis undertaken. RESULTS: A wide range of lip curve behaviour was seen in individual patients within all groups. No significant differences were found in the change in depth of lip curve amongst the various extraction and nonextraction groups. The inherent morphology of the soft tissues appeared to be the greatest determinant of lip curve behaviour with treatment. CONCLUSIONS: It would appear that neither premolar extraction nor nonextraction treatment, if based on appropriate diagnostic criteria and followed with competent clinical management, will necessarily lead to direct effects on lip curvature.