Pleural effusions of any type are fairly common in the canine species and can be due to many causes. Clinical signs seen are a result of the effusion preventing normal respiration, independent of the underlying cause. However, the underlying cause may be investigated to determine the degree of treatment necessary as well as prognosis. The effusion fluid itself must be evaluated and in order to diagnose the origin of the pleural fluid. Neoplasia is often a cause of pleural effusion. Primary pulmonary neoplasia is an infrequent occurring neoplasia in dogs, although the incidence has increased over the last 20 years. The etiology is typically unknown but usually originates from the terminal bronchioles and alveoli. There is a wide variety of clinical presentations possible with primary lung tumors but when the development of effusion occurs, more specific signs will be seen. Presentation and diagnostic work-up of a nine year old male castrated Golden Retriever will be used to illustrate the importance of pleural effusion analysis in relation to possible disease diagnosis and prognosis.