A patent upper airway is essential for survival. Increased age, obesity and some upper airway anatomical features are associated with failure to maintain upper airway patency during sleep, leading to obstructive sleep apnoea. However, many healthy subjects with these risk factors do not develop this condition. The aim of this study was to determine how anatomical factors and active dilatormuscle contraction contribute to upper airway patency in healthy volunteers across a broad range of age and body mass index (BMI). A 'tagged' magnetic resonance imaging technique quantified respiratory-related motion of the anterior and lateral walls of the upper airway during quiet breathing in the supine position. Fifty-two subjects aged 22-68 years with BMI from 17.5 to 40.1 kg m-2 were studied. Higher BMI was associated with smaller airway cross-sectional area at the level of soft palate (P<0.05). The genioglossusmoved anteriorly to dilate the upper airway during inspiration. This movement increased with increasing BMI, increasing age, a smaller airway area, and steeper tongue-base angle (all P < 0.05). Motion of the lateral upper airway at the soft-palate level was variable and less strongly linked to anatomical features of the upper airway.Multiple regression indicated that anterior genioglossusmotion decreased with increasing airway area (P = 0.03) and with increasing tongue-base angle (P = 0.02). These data suggest that healthy humans, including those whose anatomy places them at increased risk of airway closure, can maintain upper airway patency by dynamically dilating the airway during inspiration.