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INTRODUCTION Spirometry plays an important role in the assessment of possible respiratory failure in children with neuromuscular diseases (NMDs). However, obtaining reliable spirometry results is a major challenge. We studied the relation between oscillometry and spirometry results. Oscillometry is an easy, non-invasive method to measure respiratory resistance R and reactance X. We hypothesized an increased R and reduced 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 R and X at 5, 11 and 19 Hz and (Forced) Vital Capacity, Peak Expiratory Flow. Spearman correlation coefficients and positive and negative predictive values were calculated. Regression curves were estimated. RESULTS We included 148 patients, median age 13 years (IQR 8-16). A negative correlation was found between R and spirometry outcomes (spearman correlation coefficient (ρ) -0.5 to -0.6, p < 0.001). A positive correlation was found between X (i.e. less negative outcomes) and spirometry outcomes (ρ 0.4 to 0.6, p < 0.001). Highest correlation was found at lower frequencies. Regression analysis showed a non-linear relation. Measurement of inspiratory and expiratory R and X did not provide added value. Positive predictive values of 80-85% were found for z-scores of R measured at 5 Hz versus (F)VC≤ 60%. CONCLUSION We found a non-linear relation between oscillometry and spirometry results with increased R and reduced X in patients with more restrictive lung function decline. Given the difficulties with performing spirometry, oscillometry may be a promising substitute. This article is protected by copyright. All rights reserved.