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Background: Paranasal sinus cysts (PSC) are a common cause of equine secondary sinusitis. The outcome and associated complications have not been frequently reported., Objectives: To review the associated clinical signs, associated morbidities and outcomes of horses treated for PSC., Study Design: Retrospective multicentre case series., Methods: Retrospective analysis of case records and telephone follow up survey., Results: Subjects were 37 horses 1-24 years old that were presented with nasal discharge (n = 31), facial swelling (n = 25) and epiphora (n = 19). Radiography and computed tomography allowed identification of the cyst-induced changes including concomitant tissue destruction (n = 31), leading among other things to local nerve damage causing headshaking (n = 6) and unilateral blindness (n = 1). Radiographic changes to adjacent dental apices were present in 10 horses. Horses over 10 years old showed more of the named associated problems. Post-operative complications included surgical site infection (SSI) (n = 11), nasofrontal suture periostitis (n = 6) and sequestration (n = 1) following removal of the PSC via osteotomy. The long-term response to treatment was available for 28 cases with 22 horses (78.6%) fully cured, 4 (14.3%) partially cured and 2 (7.1%) not responding to treatment. In 7 horses (18.9%) there was recurrence of the cyst post-operatively., Main Limitations: Due to the study being a multicentre retrospective case series with collection of data over an extended period, there may be inconsistency in data recording and absence of reporting of some findings., Conclusions: Overall, the diagnosis and treatment of sinus cysts is relatively straightforward and carries a good prognosis. In long-standing cases complications secondary to the expansive growth of cysts will dramatically affect the prognosis for full recovery due to pressure-induced changes to facial bones, cheek teeth and nerves. These secondary complications mainly occurring in older horses may be due to a combination of a relatively longer period of affection and the inflexibility of older horses' bones. Cyst recurrence following treatment can occur in up to 19% of cases., (© 2018 EVJ Ltd.)