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Purpose Lung transplant (LTX) recipients at Royal Papworth Hospital are routinely followed-up with lung function and chest X-ray. Acute cellular rejection (ACR) is a major complication, which is currently a significant risk factor for chronic lung allograft dysfunction. ACR is generally identified by routine monitoring with spirometry, however, impulse oscillometery (IOS) may provide a more detailed assessment of lung function, independent of patient effort. This study aimed to investigate the association between IOS and spirometry parameters cross-sectionally and longitudinally, with particular focus on those with a clinically significant decline in FEV1.Methods All post LTX patients attending for routine monitoring of spirometry between the 1st of December 2023 and the 1st of October 2024, performed IOS. Analyses were performed to investigate the correlation between IOS parameters and FEV1 at baseline, and longitudinally thereafter. Furthermore, the differences in IOS parameters among those patients who did vs did not have a 10% decline in FEV1 during the follow-up period were compared by Mann Whitney-u test.Results The LTX cohort consisted of 213 patients (female 38%, median age 57). IOS parameters correlated most strongly with FEV1, of which moderate correlations were demonstrated for R5-R20, X5 and AX (r= -0.57, 0.65 and -0.59 respectively p<0.001). In total, 93 patients had at-least one follow-up IOS and spirometry during the study. A moderate correlation was observed between change in FEV1 and change in R5, X5, AX and R5-R20 over time (r= -0.49, 0.60, -0.68 and -0.66 respectively p<0.001). All IOS parameters except for R20, were significantly more impaired among those with a 10% decline in FEV1, compared to those who did not have a 10% decline in FEV1 during the follow-up period.Conclusion IOS parameters indicative of small airways changes (R5-R20%, X5 and AX) are correlated with FEV1 both cross-sectionally and with respect to change over time. This highlights the potential benefit of using IOS alongside standard spirometric measurements in this patient cohort, to detect early changes and aid in earlier intervention.