Objectives:To assess the estimation of crowding by orthodontists and their subsequent extraction choices.Materials and methods:Sixty-two orthodontists were asked to quantify crowding on eight lower arch study models using their preferred method and also to indicate possible extraction choices. For each model, the intermolar widths, intercanine widths, and clinical scenarios were identical, but the true crowding varied from 0.1 to 7.5mm across the eight models as to a lesser extent did the curve of Spee. Eleven orthodontists repeated the exercise after 9 months to assess reliability.Results:The preferred method of space estimation by all of the orthodontists was direct visualization. However, the estimates of crowding were very variable. For the most crowded lower model with 7.5mm of crowding, the estimates ranged from 5 to 20mm. Extraction choices were less variable than estimates of crowding and shifted from second to first premolars as crowding increased. Estimates of crowding and treatment decisions changed with time in 28 of 33 repeat measures. Estimates of crowding were unrelated to clinical experience.Limitations:The principal limitation of this study is that it was a laboratory-based study and utilized just the lower arch model for estimation and treatment planning.Conclusions:Extraction decisions and estimates of crowding tended to vary both initially and over time but were less varied in the case of the extraction decisions. Although this may have been a reflection of the limited treatment options, perhaps reassuringly, as the degree of crowding increased, so did the likelihood of prescribing extractions and the decisions generally shifted from second to first premolars. How orthodontists estimate crowding and make subsequent extraction choices is important and has potential medico-legal implications.