A 15-year-old domestic cat presented with restrictive dyspnoea and pyrexia. Thoracic radiographs and computed tomography (CT) revealed a caudal mediastinal paraesophageal empyema associated with pyothorax, pleuritis and pericardial effusion. No foreign body was visible. The bacteriology was positive for Fusobacterium nucleatum. Medical treatment was initiated and consisted of bilateral drainage/lavage of the pleural cavity and intravenous amoxicillin/clavulanic acid followed by oral administration.A second CT scan performed on Day 7 showed regression of the lesion and confirmed the effectiveness of the medical treatment. Thoracic drains were removed on Day 8, and antibiotic treatment stopped 3 weeks postdiagnosis. A CT scan performed at 6 weeks postdiagnosis revealed complete regression of the lesions.