INTRODUCTION: Although lip bumpers (LBs) provide significant clinical gain of mandibular arch perimeter in mixed-dentition patients, orthodontists are reluctant to use them due to the possibility of permanent second molar eruptive disturbances. OBJECTIVE: The present study was conducted to assess second molar impaction associated with the use of LBs, and to investigate how they can be solved. MATERIAL AND METHODS: Lateral and panoramic radiographs of 67 patients (34 females and 33 males) were assessed prior (T1) and post-LB treatment (T2). LB therapy lasted for approximately 1.8 ± 0.9 years. Concomitant rapid palatal expansion (RPE) was performed in the maxilla at LB treatment onset. Impaction of mandibular second molars was assessed by means of panoramic radiographs in relation to the position of first mandibular molars. Horizontal and vertical movements of first and second molars were assessed cephalometrically on lateral cephalometric radiographs based on mandibular superimpositions. RESULTS: Eight (11.9%) patients had impacted second molars at the end of LB therapy. Two patients required surgical correction, whereas five required spacers and one patient was self-corrected. Mandibular first molar tip and apex migrated forward 1.3 mm and 2.3 mm, respectively. Second molar tip showed no statistically significant horizontal movement. CONCLUSION: Although LB therapy increased the risk of second molar impaction, impactions were, in most instances, easily solved.