Background There is little objective information concerning the intra‐ and post‐operative complications or the long‐term outcome of sinoscopic treatment of equine sinus disorders. Objectives To document the long‐term outcome, including intra‐operative complications, reasons for treatment failure and other complications, in horses undergoing standing sinoscopic treatment of sinus disorders. Study design Retrospective clinical study. Methods Records of sinus disease cases presented to The University of Edinburgh Veterinary School between January 2012 and July 2019 were reviewed. Follow‐up information was obtained from clinical records and a telephone questionnaire. Results Long‐term follow up for 155 cases treated sinoscopically showed that 108/155 (69.7%) fully responded to their initial treatment. Concurrent intranasal lesions were identified in 37.4% of cases. Sinusotomy was later required in 10 cases to improve surgical access or sinonasal drainage. Reasons for failure to fully respond to the initial treatment (n = 47) included: intra‐sinus bone sequestra (n = 9), inspissated exudate (n = 6) or insects (n = 2); similar material and/or infected conchal bullae in the middle meatus (n = 7); persistent oro‐maxillary fistulae (n = 4), misdiagnosed dental apical infections (n = 4); impaired sinonasal drainage (n = 4), progressive ethmoid haematoma regrowth (n = 3) and undiagnosed causes (n = 5). Further treatment of 43 of these cases (67% as outpatients) showed 34/43 cases fully responding to their second treatment. Only 4/155 cases (2.6%) required sinonasal fenestration. In the long term, 149/155 cases (96.1%) showed full and 2/155 cases (1.3%) showed partial improvement. Sinoscopy portal wound infection occurred in nine cases. Main limitations The length of time between treatment and obtaining follow‐up information in some cases. Advances in knowledge and techniques over the duration of the study. Conclusions Sinoscopic treatment is a minimally‐invasive technique causing minimal morbidity and was successful in 96.1% of cases. Meticulous care should be taken to ensure that bony sequestrae and inspissated exudate are fully removed from the sinuses and nasal cavity during sinoscopic treatments.

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