The records of 55 horses with paranasal sinus disease that were admitted to the Equine clinic, University of Zurich in the years from 1996 to 1999 were reviewed. The horses were 26 mares, 28 geldings and 1 stallion of different breeds aged between 4 and 30 years (10.9±5.3). Physical examination, rhinoscopy, diagnostic imaging, treatment and prognosis were evaluated. Dental disease (n=34) was the most common cause of chronic sinusitis. The typical clinical sign of sinusitis was nasal discharge in 52 cases that was unilateral in 49 cases. In 29 horses the nasal discharge was purulent and had a fetid odor resulting from dental disease in 28 cases. With radiography in 47 cases a fluid line could be visualized in 1 or more sinuses. Signs of dental involvement in 1 or more tooth roots were suspected in 39 cases. However, only in 20 cases it could be diagnosed with certainty based on the radiographs. 25 horses were treated conservatively. Of these horses 5 had to undergo surgery after conservative treatment had failed. Altogether 21 horses underwent surgery. One tooth had to be removed in 13 cases and 2 teeth in one case. They were M1 (n=8), PM4 (n=5), M2 (n=1) or PM3 (n=1). Postoperative complications were common and consisted of chronic sinusitis (n=5), draining tracts (n=4), oral fistula (n=4) or facial wound dehiscence (n=2). Because of complications 6 horses required on additional surgical procedure. 26 horses that were treated for either primary sinusitis or sinusitis caused by dental disease were available for follow-up after 2 months. At this time 18 horses revealed no clinical signs of sinusitis.