Three-dimensional (3D) computational fluid dynamic (CFD) analysis was used to model the effect of collapsing airway geometry on internal pressure and velocity in the pharyngeal airway of sedated obese children with and without obstructive sleep apnea syndrome (OSAS). Geometry was reconstructed from volume-gated magnetic resonance images during normal tidal breathing of the respiratory cycle and solved using flow data averaged over 12 consecutive breathing cycles. In the OSAS subject, collapse initiated in the proximal nasopharynx and continued downstream into the oropharynx, while the control experienced negligible collapse. Tube laws (pressure vs. cross-section area) derived for the nasopharynx and oropharynx, indicated the oropharynx in the OSAS subject more compliant than the nasopharynx (1.028 mm2/Pa vs. 0.449 mm2/Pa) and had a lower theoretical limiting flow rate, confirming the oropharynx as the flow-limiting segment of the airway in this subject. This new method may help to differentiate anatomical and functional factors in airway collapse.