Lip bumper treatment has been shown to successfully increase arch width, procline the incisors, and distalize molars. However, few studies have been performed showing the long-term stability of lip bumper treatment. In this study, mandibular casts taken by a single practitioner from 51 patients treated with lip bumpers without rapid palatal expansion were analyzed at pretreatment, post-lip bumper treatment, posttreatment, and long-term out of treatment. Measurements of arch width, arch depth, arch length, and anterior crowding were made. During treatment, there was a mean decrease in irregularity of 3.73 mm, with a posttreatment increase of 0.76 mm, for a net decrease of 2.97 mm. Despite posttreatment decreases, significant gains in arch width were maintained for extended periods of time. The intercanine width had a net increase of 1.78 mm (19% relapse), first premolars 3.39 mm (26% relapse), second premolars 2.58 mm (34% relapse), and first molars 2.17 mm (20% relapse). Lip bumper treatment along with fixed appliances is an effective means to obtain long-term increases in arch width and decreases in the irregularity index.