Pericardial cysts occur rarely, with an incidence rate of 1 per 100,000. They are usually detected by chance and clinically silent in most cases. Pericardial cysts are the most common benign tumours of the pericardium and presents by the third or fourth decade of life, and equally common in males and females. In principle, they only require follow-up, however, an enlarging or symptomatic cyst requires surgical removal. We report a case of a 32 year-old Malay lady, who presented with history of recurrent pericardial effusion followed by right pleural effusion. Computed tomography (CT) thorax identified a large mediastinal cyst as the cause of her problem, requiring exploratory thoracotomy.