The aim of the present long-term follow-up study of orthodontically-treated patients (mean: 15.7 years) was to analyse post-retention changes and to reveal factors which may play a role as predictors for long-term prognosis. Pretreatment, end-of-treatment, and post-retention models of 226 cases with all types of anomaly were used to measure intercanine and intermolar width, arch length, and sum of the mesiodistal dimension of the incisors, Irregularity Index, crowding, molar and canine relationship, overjet, and overbite. In order to assess the influence of sex, initial and end-of-treatment alignment, type of therapy, amount of tooth movement, and presence of third molars on the extent of post-retention changes, the total sample was divided into subgroups. Findings indicated that post-retention crowding and incisor irregularity increased more frequently in the mandible than in the maxilla. Pretreatment variables such as increased mesiodistal incisor dimension, severe crowding and incisor irregularity, arch length deficiency, arch constriction, and increased overbite as well as post-treatment spacing, arch expansion, increased arch length, and residual Class II or III molar relationships were found to be associated factors in the process of post-retention increase of crowding and incisor irregularity. 'Overexpansion' was found to be a factor in mandibular incisor relapse.