The aim of the study was to analyse the day-to-day variability of the respiratory resistance (R-rs) and the reactance (X-rs) in 5 horses in a status of remission of recurrent airway obstruction by forced oscillometry system (FOS). Furthermore, the effects of stabling, outdoor and room temperature and humidity on these parameters were determined. Mean R-rs at oscillation frequencies 1, 1.5, 2 and 3 Hz were all significantly higher in the morning than in the afternoon, while X-rs was not significantly affected by time of the day. R-rs was significantly different on various sampling days at all frequencies, in the morning as well as in the afternoon. X-rs showed the same significant differences for frequencies greater than 1 Hz. Overall, R-rs showed a smaller variability than X-rs. Neither outdoor temperature, nor room temperature affected R-rs. This also applied to X-rs except for the measurements at 2 Hz that were significantly (p = 0.044) affected by outdoor temperature. Turning out these horses at paddock for day and night caused a significant lower mean R-rs that when kept at stable for all night. While R-rs showed a very weak positive frequency dependency, X-rs clearly showed a positive frequency dependance for all horses. Resonance frequency (f(res)) was between 2 and 4 Hz. These studies showed that FOS generated parameters are affected by environmental and management factors and thus only after well-designed standardized procedures FOS could be a useful diagnostic tool for the diagnosis and monitoring of equine respiratory disorders.