Introduction: The purpose of this retrospective study was to identify (and quantify the effect of) factors that influence orthodontic treatment time. Method: The sample consisted of 366 consecutive orthodontic patients (220 female, 146 male, aged 10-20 years, treated by 1 orthodontist) who completed treatment in a single stage with fixed appliances. Four categories of data (sociodemographic characteristics, malocclusion characteristics, treatment methods, and patient cooperation) were collected from treatment records and analyzed. Results: Average treatment time was 23.5 months (range, 12-37; SD, 4.7). A multiple regression model explained 38% of the variance and identified 9 significant variables. Five were pretreatment characteristics: male sex, maxillary crowding of 3 mm or more, Class II molar relationships, proposed treatment plan involving extractions, and delayed extractions. The remaining variables (3 of which were associated with patient cooperation) were poor oral hygiene, poor elastic wear, bracket breakages, and brackets rebonded for repositioning. Conclusions: Orthodontic treatment time is influenced by a number of patient characteristics and clinical decisions. It is possible to predict estimated treatment time for a patient by using a small number of personal characteristics and treatment decisions.