Polyserositis (PS) is the inflammation, with effusion, of different serous membranes. It has been associated with different aetiologiesbut the aetiology of PS remains unknown in a high percentage of patients. The general objective of this retrospective study was to analyse the aetiology of PS cases seen at Son Llàtzer Hospital in an eleven-year period. Other objectives were to determine epidemiological, clinical, and analytical characteristics of these patients. Ninety-two patients were included in the study. The most common diagnosis was neoplasm (nearly one third of cases) followed by infectious and autoimmune diseases. PS aetiology was unknown in more than one third. Pleura and pericardium were the most common sites of serosal involvement (83%). Antinuclear antibodies positivity in serum and increased levels of adenosine deaminase in pleural effusion were significantly associated with a final diagnosis of autoimmune disease. Increased pleural lactate dehydrogenase levels were significantly associated with a final diagnosis of neoplasm. In 9/14 patients with a previous cancer, PS represented a recurrence of their cancer. Cases of unknown aetiology presented most frequently as pleural and pericardial involvement and the majority resolved. In very few an infectious etiology could be proven. The most important limitations of the study are its retrospective design and the lack of a systematic work-up and a diagnosis algorithm for PS. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.