The purpose of this study was to compare post-treatment stability in Angle Class III cases grouped by treatment modality into extraction, non-extraction, or surgical. Twenty-seven Angle Class III patients (9 men, 18 women) were selected from among the records of qualified retention cases treated at the Department of Orthodontics, Tokyo Dental College. The patients were divided into the following groups: Group 1, extraction (n=10); Group 2, non-extraction (n=9); and Group 3, surgery (n=8). Lateral cephalograms, panoramic X-rays, and study casts were obtained at baseline (T0), after active treatment (T1), and after retention (T2). The ABO Model Grading System (ABO) and Gottlieb's Grading Analysis (GGA) were used as indices of occlusion, and the scores at each stage and differences in scores at T1 and T2 (|T1-T2|), defined as post-retention stability, recorded. Little's Irregularity Index (IRI) from the study casts and the ANB from lateral cephalograms at T0, T1, and T2 were also recorded to assess post-treatment stability from a different perspective. The Kruskal-Wallis test was performed for statistical comparisons. Mean ranking according to the ABO and Kruskal-Wallis test at T1, T2, and |T1-T2| was lowest in Group 1 and highest in Group 3, and the mean amount of change in the ABO score after active treatment was also the same. This suggests that post-treatment stability is highest in non-extraction cases and lowest in surgical cases. This result correlated with change in the ANB from T0 to T1. This indicates that amount of change in the ANB affects post-treatment stability and that relapse will tend to be greater in surgical cases. However, the amount of change in IRI after active treatment was highest in Group 2 and lowest in Group 3, suggesting that relapse of the lower incisors is greatest in non-extraction cases.