One of the first questions asked by new orthodontic patients is: How long will I need to wear my braces? A multitude of factors have the potential to influence the answer to this question. The purpose of this retrospective study was to identify some of the primary factors which influence orthodontic treatment duration. Few studies have attempted to evaluate these factors previously. Data were gathered from 140 consecutively completed, comprehensive treatment patient records in five orthodontic offices. Thirty-one variables related to patient characteristics, diagnostic factors, modality of treatment, and patient cooperation were evaluated. Permanent teeth were extracted in 24.3% of the cases, and 12.1% were treated in more than one phase. Average treatment time was 28.6 months with a range of 23.4 to 33.4 months among the five offices. Nearly half (46.9 percent) of the variation in treatment duration was explained by a five-step multiple regression analysis. By comparison, previous studies using a similar regression analysis have explained 25% to 33% of the treatment time variation in their samples. Included in the regression equation were 1) the number of missed appointments, 2) the sum of replaced brackets and bands, 3) the number of treatment phases, 4) the number of negative chart entries regarding oral hygiene, and 5) whether or not headgear wear was prescribed during treatment. A second regression analysis was then run to examine the additional contribution of between office differences to the explanation of variation in treatment time. An additional 6.7% of the variance was explained by variation among the five offices. Even though a total of 53.6% of the variance in the duration of orthodontic treatment could be explained by multiple regression analysis, a significant portion was left unexplained. Six of the thirt-one variables examined made a statistically significant ([alpha]= 0.01) contribution to the explanation of variation in treatment time. The quality of the finished cases and the appropriateness of the original diagnosis and treatment plan were not evaluated. Developing an objective assessment to evaluate these areas may be important for increasing our understanding of treatment time. The quality of the finished cases and the appropriateness of the original diagnosis and treatment plan were not evaluated. Developing an objective assessment to evaluate these areas may be important for increasing our understanding of treatment time variation in the future.