It is a well-established fact that arch length is lost during the transition from the mixed to the permanent dentition, particularly in the mandibular arch. One estimate is that the average mandibular arch loss is 1.8 mm. Since this reduction, bilaterally, represents 3.6 mm of arch perimeter, the question arises: in patients with crowding in the mixed dentition, can simple arch length maintenance during the transition period provide adequate space to prevent crowding in the permanent dentition? The results of 2 studies indicate that the answer to this question is yes. In the first study, 100 mixed dentition models of the mandibular arch were analyzed. (Only mandibular arch conditions were evaluated because they generally dictate the strategy for maxillary arch treatment.) A conventional tooth size-arch size assessment was performed to quantify the crowding. The mesiodistal diameters of the teeth were measured, and a brass wire was extended from the mesial of a first molar to its antimere, adapted over the fossae of the posterior teeth and the cusp tips of the canines, and idealized in the anterior segment between the canines. Crowding, present in 85 of the 100 patients, averaged 4.39 3.39 mm (range, 4.46-13.46 mm). The sizes of the unerupted permanent teeth were derived nonradiographically by using ratios of primary to permanent teeth to determine the leeway space that would become available by maintaining arch length. The leeway space was 5.15 0.68 mm (range, 1.6-7.64 mm). When the leeway space was compared individually to crowding, there was adequate space to resolve crowding in 62 (72%) of the patients. Of the remaining 23 patients, 7 had less than 2 mm of crowding, and 16 had 2 mm or more. The relationship between early loss of a primary canine and crowding was also evaluated. Nineteen subjects lost a primary canine prematurely, and 18 of these patients had crowding. Only 7 of the 18 (39%) had adequate space to resolve the crowding when the leeway space was included. In contrast, when crowding was not associated with the early loss of a primary canine, the leeway space proved adequate to correct the crowding in 82% of the patients. As expected, the early loss of a primary canine combined with crowding results in a more crowded dentition. This study indicated that the space to resolve crowding in the mixed dentition could be obtained in most patients simply by maintaining arch length during the transition from mixed to permanent dentition, but these results are theoretical and should be tested clinically. The second study was a clinical investigation involving the evaluation of the outcomes of arch length maintenance by means of passive lingual arches during the transition period in 107 consecutive patients. Tooth size-arch size discrepancies were measured as previously described at 2 times—in the mixed dentition and in the early permanent dentition with the second premolars at least 50% erupted. Total arch length, representing the combined sum of right and left distances from the mesial contact points of the first molars to the contact points between the central incisors, and the widths between canines, premolars, and molars were also measured. Even though lingual arches were placed, total arch length decreased 0.44 mm ( 0.17 mm on the right and 0.27 on the left). Arch length decreased in 62 subjects, increased in 39, and remained the same in 6. Arch width increased during the transition from mixed to permanent dentition. Intercanine width increased by 1.49 mm, interfirst premolar width increased by 2.27 mm, and intermolar width increased by 0.72 mm. The increase in intercanine width, though larger than noted in untreated subjects, is similar to that reported by DeBaets and Chiarini (1.1 mm) after Goldman School of Dental Medicine, Department of Orthodontics, Boston University, Boston, Mass. Presented at the International Symposium on Early Orthodontic Treatment, February 8-10, 2002; Phoenix, Ariz. Am J Orthod Dentofacial Orthop 2002;121:569-71 Copyright © 2002 by the American Association of Orthodontists. 0889-5406/2002/$35.00 0 8/1/124172 doi:10.1067/mod.2002.124172