AIM: to evaluate the effects of rapid maxillary expansion (RME) in a group of OSAS preschool children. MATERIALS AND METHODS: Lateral cephalograms of 15 OSAS children (8 boys and 7 girls, age mean ± SD: 5.94 ± 1.64 years) were analysed at the start of treatment with RME (T0). All subjects were revaluated after a mean period of 1.57 ± 0.58 years (T1). At this time the sample was divided into 2 groups according to the change in the respiratory disturbance index (RDI): an improved group (I: 8 subjects) and a stationary/worsened group (SW: 7 subjects). Differences between I and SW children with respect to values of cephalometric variables at T0 and to variations between T0 and T1 were evaluated using Mann-Whitney U test. Differences between T0 and T1 values in the overall group of children and separately in I and SW groups were assessed using Wilcoxon test. RESULTS: At the start of treatment, the I group was characterised by more retrognathic jaws with lower values of SNA (p=0.055) and SNB (p=0.020) and higher age values (p=0.093) when compared to SW group. After treatment, the I group showed an increase in SNA and SNB angle significantly higher than SW group (p=0.004 and p=0.003, respectively). On the contrary, I and SW groups did not differ as for variation in the skeletal divergency and in the total facial height. CONCLUSION: OSAS preschool children with retrognathic jaws could benefit from RME treatment.