A five year old Swiss Warmblood gelding was presented with unilateral, chronic, mucopurulent nasal discharge. Clinical, radiographic and computertomographic examinations showed a mass in the ventral meatus of the nasal cavity. The mass had connection to the root of the fourth premolar tooth. During surgery, the fourth premolar tooth and the mass were completely removed. A rectangular bone flap of the maxillary bone was performed to gain access to the apex of the tooth. The fourth premolar tooth had to be repelled initially before the neoplasia could be isolated and removed through the surgical approach. The large opening between the oral and the nasal cavities was closed with a silicon-dental packing. The histologic examination of the neoplastic tissue revealed an ostelipoma. Eight weeks later the silicon-plug was removed orally. Four months after the surgery the horse was presented again with an unilateral, purulent nasal discharge. In the mean time a fistula developed between the oral cavity and the maxillary sinus. Clinically as well as radiographically the patient exhibited all the signs of a chronic sinusitis of the maxillary and frontal sinus. In a second surgery the fistula was closed with a methylmethacrylat-plug. After another 2 months the horse was presented without any complications. The methylmetacrylat-packing was removed 3 months later.