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 dependence 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.