OBJECTIVE To analyse the maxillary arch dimensions of children aged 9 and 12 with unilateral cleft lip and palate (UCLP) who received orthodontic treatment and secondary alveolar bone grafting. METHODS This retrospective cohort study was performed on 30 patients with UCLP. METHODS All patients were treated at the Cleft Lip and Palate Centre, which is part of the University Medical Centre Amsterdam and Academic Centre for Dentistry Amsterdam, the Netherlands. METHODS Children with non-syndromic UCLP who received pre- and postsurgical orthodontics combined with secondary alveolar bone grafting between the ages of 9 and 12 years were included. METHODS Maxillary arch dimensions were assessed on 60 digitised dental casts with measurements of the intermolar widths, interpremolar widths, arch perimeters, arch lengths, arch widths, and palatal depths. RESULTS The results of a paired-samples t-test revealed a statistically significant increase (P < .05) in intermolar width 1, intermolar width 3, interpremolar width 1, arch perimeter, and arch width between the ages of 9 (T0) and 12 (T1). Intermolar width 2 and the palatal depth decreased statistically significantly between T0 and T1. CONCLUSIONS Analysis of maxillary arch dimensions of children with UCLP indicates significant changes between 9 and 12 years of age. This suggests that orthodontic treatment and secondary alveolar bone grafting can be effective in improving maxillary arch dimensions. However, there is a need for collaborative research and data collection in order to provide sensible and evidence-based care to patients with cleft lip and palate.