Study Objectives: The goal of this study was to determine how high frequency inspiratory sounds (HFIS) are generated by sleeping children with obstructive sleep disordered breathing (OSDB). We hypothesized that HFIS are generated when a high velocity jet of air, generated by a narrowed upper airway, induces the upper airway to act as a resonating chamber. We tested two predictions of this hypothesis: 1) the upper airway is narrowed in children who make HFIS and 2) the length of the upper airway, calculated from HFIS harmonic intervals, is similar to that calculated from magnetic resonance imaging (MRI) scans. Setting: Sleep Laboratory. Participants: Twenty-nine children between 6 and 12 years of age with adenotonsillar hypertrophy suspected of having OSDB. Measurements: Minimum cross-sectional airway area and airway long dimensions (lips to larynx or soft palate) were measured in awake children with MRIs. Later that night, sound was recorded with a microphone suspended above their bed while the children underwent polysomnography. Sounds were later analyzed with FFTs. Results: Sleeping children who generated HFIS had significantly narrower upper airways when compared to children who did not make HFIS (minimum airway area 20.5 ± 4.4 mm² vs. 70.9 ± 22.5 mm², respectively, mean ± 1 SEM, p = 0.02). There was a significant inverse correlation between the log₁₀ of the narrowest airway area and the number of HFIS recorded per hour (r² = 0.55, p < 0.00001). The harmonics characteristics of HFIS predicted that they were generated by sound resonating in chamber whose length was 12.0 ± 0.9 cm which is similar to the MRI measured distance from the lips to the larnyx of 12.8 ± 0.4 cm. Conclusions: These data suggest that children generate HFIS when 1) they have a narrowed upper airway and 2) when their upper airway acts as a resonating chamber.