Aim <p>To investigate the effect of Slow Maxillary Expansion (SME) on Maximum Tongue, Lip, and left and right Cheek Pressure (MTP, MLP, MCP), with the aim to identify possible correlations with dental characteristics and parafunctions.</p> Materials and methods <p>Patients with mixed dentition presenting crossbite, lack of space in the upper jaw and/or a mandibular shift were included. Digital casts were analysed and perioral muscle pressure was assessed using the Iowa Oral Performance Instrument before (T₀), after active expansion (T₁) and at the end of treatment (T₂). Additionally, parafunctions were assessed.</p> Results <p>136 children were included. SME significantly increased arch length, intermolar and intercanine distance and reduced upper anterior crowding. Sagittal molar relationship and overbite improved. MTP and MCP increased significantly at T₁ and T₂, while MLP remained unchanged. A negative correlation was observed between MLP and intermolar and intercanine distances whose significance did not maintain after correction for multiple testing. Nail/pen biting and mouth breathing decreased significantly during and after treatment. The presence of lip interposition led to higher MLP and left MCP, also non-significant after correction.</p> Conclusion <p>SME with removable appliances in mixed dentition leads to significant short-term improvement of dentoalveolar characteristics, increasing MTP and MCP and reducing the presence of parafunctions such as nail/pen biting and mouth breathing. No substantial correlation was observed between muscle pressure changes and dental changes, suggesting the latter to be primarily attributed to the appliance itself.</p> Clinical relevance <p>SME in mixed dentition leads to myofunctional adaptation and has a positive effect on parafunctions.</p>