Aim To explore the relationship between postural changes in lung function and polysomnography (PSG) in the assessment of early sleep disordered breathing in children with neuromuscular disorders [NMD]. Methods In this prospective cross-sectional study, children with NMD performed spirometry in sitting (si) and supine (su) positions. A control group of age and gender matched healthy children also underwent postural lung function testing. PSG was performed within 6 months of spirometry. Spirometry was acceptable according to ATS standards and PSGs scored according to AASM guidelines. Results Forty-one children with NMD, aged 12.3 ±3 years (21 males) performed sitting spirometry. Thirty [73%] performed acceptable spirometry in the supine position. Underlying diagnoses were heterogeneous, with the majority having Duchenne Muscular Dystrophy (n=17). Mean FEV1sit and FVCsit were 78% (SD ±22) and 75% (SD±20.4) respectively, with mean% ΔFVC (sit – sup) 9±11% (range 2% to 20%), and was significantly greater than healthy controls (n=30 SD ±3) (p<0.001). PSG data on these 30 children showed total AHI 6.9 ±5.9/hr (0.3 to 29), obstructive AHI 5.2±4.0/hr (0.2 to 10), and REM AHI 14.1+/-5.3/hr (0.1 to 34.7). A moderate correlation was present between supine FVC% and AHI (r=0.62, p=0.001) in those not using non-invasive ventilation [NIV] in sleep [N=22] but not with the rise in CO2 from non-REM to REM sleep (6 ±1.9 mmHg, range 4 to 11). Conclusion Children with NMD and mild restrictive lung disease showed greater postural changes in spirometry than healthy controls, with changes being greatest in children who required nocturnal NIV.