In this case, the use of the Cell Block technique in the diagnosis of pleural effusion collected from a 3.5 year old Setter Spaniel breed dog was investigated. The dog was presented to Ankara University Small Animal Hospital with the history of acute dyspnea, weakness, exercise intolerance and tachypnea for a week. Routine clinical examinations including blood testing were performed. Abdominal distension, pleural effusion and cyanotic cranial mucous membranes were remarkable during physical examination. The pleural fluid collected by thoracocentesis was brownish-colored, cloudy and has contained granular materials. The specimen collected from the dog was separated into three aliquots, one part was processed in a routine manner for cytological examination by cytocentrifugation, and the other parts of the sample were used for a cell block and bacteriological analysis. In microbiological analysis, Escherichia coli and Klebsiella spp. were isolated. The cytocentrifuged preparations and cell block sections revealed septic effusion. The cell block technique demonstrated morphological details better than cytocentrifugation. In conclusion, in this case, cytology and bacteriology of effusion, routine blood tests, abdominal radiography and echocardiography showed that the pleural effusion was resulted from pyothorax due to severe tricuspid dysplasia.