Subjects affected by Duchenne muscular dystrophy (DMD) develop severe malocclusions with the progression of the disease, due to changes in orofacial musculature and function, including tongue hypertrophy. We aimed to evaluate the benefits of maintaining mandibular interarch width with the help of a simple fixed orthodontic appliance. Three adolescent DMD boys were selected consecutively to receive a passive rigid mandibular lingual arch, and followed for 4-5 years. An untreated age-matched control group was chosen and followed for a similar period. Study casts were obtained at baseline and after follow-up. Outcomes measured were overjet, overbite, maxillary and mandibular intermolar widths, mandibular arch depth, molar relationships, and the presence of lateral crossbites and anterior or lateral openbites. Changes in measurements obtained between the two time points were compared in each age-matched pair. There was a clinically important increase in the mandibular intermolar width in the non-treated children ranging from 2.5 mm to 9 mm, but not in those treated. Malocclusions generally deteriorated in untreated children while they remained stable in treated children. The use of a rigid mandibular lingual arch in boys with DMD can help slow down the rapid deterioration of the developing malocclusions that accompanies the progression of the disease.