Background: The current limited evidence may be suggestive that mandibular advancement appliance (MAAs) result in improvements in AHI scores, but it is not possible to conclude that MAAs are effective to treat paediatric OSA. There are significant weaknesses in the existing evidence due primarily to absence of control groups, small sample sizes, lack of randomization and short-term results. Aim: the objective of the present study was to evaluate MAAs in children with OSA. Material and Methods: Children presenting an apnea-hypopnea index (AHI) greater than or equal to one event per hour were considered to be apneic. This group of children with AHI greater than or equal to one was randomly divided through a draw into two subgroups: half of them in an experimental subgroup and half of them in a control subgroup. In the experimental subgroup, molds of each of these children’s maxillary and mandibular arches were taken using standard molds and molding material. The control group did not use any intraoral device and did not undergo any type of treatment for OSAS. The MAAs used in this study had the aim of achieving mandibular advancement, thereby correcting the mandibular position and dental occlusion, and perhaps increasing the airway and treating OSAS. After 12 consecutive months of use of the mandibular advancement devices, polysomnography examinations using the same parameters as in the initial examinations were requested for both the experimental and the control subgroup. Results: There was a decrease in AHI in the experimental group and an increase in the control group, with statistical significance. These data were used to calculate the sample size, which was 28 children in total in the groups. Conclusions: There was a decrease in AHI one year after implementing use of mandibular advancement devices, in comparison with the group that did not use these devices.