The purposes of this study were to determine the range of likely mandibular arch dimensional and positional changes that occur with growth when a particular nonextraction treatment approach is used in the late mixed dentition and to search for factors that might affect those changes. The records (pretreatment and posttreatment lateral cephalograms and study casts) of 182 patients with Class I or Class II malocclusion, with or without crowding, were assessed prospectively. Considerable individual variation was found in all measurements, and anteroposterior dimensional arch changes were found to be significantly correlated with the amount of space required for leveling and relief of crowding in the mandibular arch. Despite those relationships, mandibular arch space requirements of 4 to 8 mm were found to be associated, on average, with small anteroposterior and transverse dimensional changes. By starting treatment in the late mixed dentition, it might be possible to control anteroposterior mandibular incisal changes reasonably well, even with considerable space requirements. Transverse mandibular arch changes in this sample were generally greater across the first premolars than across the canines, something previously acknowledged to improve stability after orthodontic treatment. A wide range of variation exists in the amount of available bilateral mandibular E-space; consequently, assessing the relative sizes of the deciduous second molars and their permanent successors in individual orthodontic patients is necessary.