This study was designed to examine lateral cephalometric and arch dimensional changes that occur in the mandibular arch during orthodontic treatment involving the extractions of various premolars. Pretreatment and posttreatment records of 73 patients were chosen at random from completed cases in the practice of one experienced orthodontist. Eighteen involved the extraction of lower first premolars, and 55 involved the extraction of lower second premolars. Of these 55, 29 involved the extraction of upper first premolars and 26 involved the extraction of upper second premolars. In the lower first premolar group, however, all 18 involved the extraction of upper first premolars. Males and females were evenly represented in the 3 subgroups. Pretreatment factors that suggested a basis for the extraction choice in this group of patients were found to include incisal overjet, molar relationship, and underlying vertical facial pattern. A wide variety of arch dimensional changes was found with different lower premolar extraction patterns. There was evidence, however, of more intermolar arch width reduction after the extraction of lower second premolars than lower first premolars. Orthodontic treatment with the extraction of premolars did not consistently cause a retrusive effect on the incisors. In fact, instances of proclination of the incisors occurred within all of the extraction groups. A large amount of individual variation in incisor and molar changes accompanied treatment involving all lower premolar extraction patterns.