Objectives: To investigate long-term stability 20 years after orthodontic treatment and theassociation with arch width changes during treatment.Materials and Methods: This retros pective study investigated 103 patients with Class I and IImalocclusions treated with fixed appliances with and without extractions. The sample was treatedby one experienced orthodontist and collected from a private orthodontic office. Dental casts wereobtained pretreatment (T1), posttreatment (T2), and long-term postretention (T3) ; they werescanned and converted to STL files. Measurements were evaluated in for the upper and lower arch:intercanine width (IC), intermolar (IM) width, Little’s irregularity index (LII).Results: There were 73 female and 30 male patients. Class I was present in 74 patients and ClassII in 29. Average postretention time was 17.2 (66.5) years after an average active retention time of3.4 (61.17) years. Extraction was performed in 55 patients while 48 received nonextractiontreatment. Bonferroni Post Hoc test showed that LII in the upper and lower arches at T1 wassignificantly higher in the extraction group (P , .001). Upper and lower arch LII at T3 was slightlyhigher in extraction cases but remained under 2.05 mm. LII at T3 in the upper and lower archesshowed negative correlation with IM T3 in the upper arch (Pearson, N ¼ 103, P ¼ .047), while IC inthe upper and lower arches at T3 correlated with IM T3 in the upper and lower (N ¼ 103, P , .001).Conclusions: Clinically relevant long-term stability in both arches was found in extraction andnonextraction cases. Intermolar width and its change during orthodontic treatment was aninfluential factor on long-term stability in extraction cases.