Introduction Spirometry plays an important role in the assessment of possible respiratory failure in children with neuromuscular disorders (NMDs). However, obtaining reliable spirometry results is a major challenge. We studied the relation between Forced Oscillation Technique (FOT) and spirometry results. FOT is an easy, non-invasive method to measure respiratory mechanics, i.e. respiratory resistance R and respiratory reactance X. We hypothesized an increased resistance R and reduced reactance X in patients with more reduced lung function. Methods In this prospective single center study we included all children with NMDs able to perform spirometry. We consecutively measured respiratory resistance R and reactance X at 5, 11 and 19 Hz and (Forced) Vital Capacity, Peak Expiratory Flow. Spearman correlation coefficients were calculated and regression curves were estimated. Results We included 148 patients, with a median age of 13 years (IQR 8-16). All correlation coefficients were statistically significant with p = 0.000. A negative correlation was found between resistance R and spirometry outcomes (spearman correlation coefficient (ρ) between -0.5 and -0.6). A positive correlation was found between reactance X (i.e. less negative outcomes) and spirometry outcomes (ρ between 0.4 and 0.6). Highest correlation was found at lower frequencies. Regression analysis showed a non-linear relation between FOT and spirometry results. Conclusion We found a non-linear relation between FOT and spirometry results with increased resistance R and reduced reactance X in patients with more restrictive lung function decline. Given the difficulties with performing spirometry, FOT may be a promising surrogate measure of lung function.