We present the case of a 55-year-old man with a self-limiting febrile condition associated with polyserositis with an inconclusive investigation. Bilateral pleural and pericardial effusions resolved. The peritoneal fluid loculated and was compatible with an exudate. The patient remained clinically asymptomatic. However, 2 years later, examination revealed a palpable and painless abdominal mass, which imaging suggested a cystic lesion. Surgical resection was performed and histological examination identified a mesenteric pseudocyst. Mesenteric pseudocysts are rare intra-abdominal cystic masses which are mostly benign and do not cause specific symptoms. Although imaging tests are useful for their differential diagnosis, histology is mandatory. LEARNING POINTS• Mesenteric pseudocysts are rare intra-abdominal cystic masses with a clinical presentation that sometimes is a diagnostic challenge.• Abdominal imaging used to investigate a concomitant systemic illness may reveal a fluid lesion which may delay the diagnosis.• Ultrasound, computed tomography and magnetic resonance imaging can be used for pre-operative diagnosis, but exact differentiation is only achieved through histopathological examination. KEYWORDSMesenteric pseudocyst, peritoneal fluid, abdominal cyst. CASE REPORTThe authors describe the case of a 55-year-old man, a smoker (15 pack-years) with no other previous relevant diseases who presented with a 3-week history of dyspnoea, orthopnoea and dry cough. Examination showed low grade fever (37.8°C) and bilateral decreased breath sounds. Thoracic radiography showed blunting of both costophrenic angles, and a discrete infiltrate in the inferior right pulmonary lobe. Thoracentesis revealed an effusion with exudate characteristics and with 240/mm 3 predominantly mononuclear cells. Microbiological cultures and also direct examination and cultures for tuberculosis were negative. The patient was admitted under the diagnostic hypothesis of pneumonia, and amoxicillin/clavulanic acid and azithromycin were initiated. Further echocardiographic investigation showed a discrete pericardial effusion with no other alterations. Abdominal ultrasound revealed perihepatic and perisplenic fluid. Blood tests revealed high levels of inflammatory markers but no other relevant findings. The patient was afebrile on the second day of admission and became asymptomatic with full symptomatic recovery.

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