Mucinous cystadenoma is a rare benign neoplasm and is usually discovered incidentally. Pleuritis and pericarditis, inflammation of the pleura and pericardium, may represent manifestations of autoimmune disorders especially in female subjects. We report a patient with polyserositis that was resolved after removal of the mucinous cystadenoma. To the best of our knowledge, this is a first report describing pleuritis and pericarditis as an initial presentation of mucinous cystadenoma of an appendix. A forty-year-old Caucasian female patient with a history of pleuritis and recurrent pericarditis was admitted to the hospital due to acute abdomen. At that time she was taking indomethacin and colchicine due to pericarditis that was controlled only with the combination of these two drugs. The patient had elevated erythrocyte sedimentation rate (ESR), increased C-reactive protein (CRP) and normocytic anemia. Immunological tests, including antinuclear antibody, anti-neutrophil cytoplasmic antibody, rheumatoid factor, and anti-cyclic citrullinated peptide antibodies, were repeatedly negative. Emergency surgery revealed acute appendicitis with perforation and subsequent diffuse peritonitis. Histopathological examination showed acute appendicitis and mucinous cystadenoma. Following the surgery the patient did not take any drugs. Fourteen months later the patient was symptom free. Pleuritis and pericarditis in female patients are most often associated with autoimmune diseases. We assume that increased ESR and CRP with anemia detected in the patient may reflect the altered immunity that is due to mucinous cystadenoma. We believe that this report has a broader clinical impact, implying that benign tumor could alter immunity, which can lead to unusual presentation such as polyserositis.