This study assessed the effect on the upper airway during sleep nasendoscopy of mimicking the action of a mandibular advancement splint. Twenty-seven subjects with a diagnosis of sleep-disordered breathing were referred for mandibular advancement splint therapy following sleep nasendoscopy. Sleep nasendoscopy was repeated for all subjects with, and without, the appliance in situ. Follow-up sleep studies with a mandibular advancement splint in situ were undertaken for 19 individuals with significant obstructive sleep apnoea. With the mandibular advancement splint, subjective snoring levels and airway patency improved as predicted in all but one individual. Residual palatal flutter was predicted for five subjects and occurred in eight individuals. Follow-up sleep studies showed highly statistically significant reductions in median apnoea-hypopnoea index (from 28.1 to 6.1, p < 0.001). Mimicking the action of a mandibular advancement splint during sleep nasendoscopy helps considerably in the patient selection process for this form of treatment.