Patient: Female, 58 Final Diagnosis: Pericardial effusion and mass Symptoms: Fatigue • lower extremity edema • shortness of breath Medication: — Clinical Procedure: Pericardiocentesis Specialty: Rheumatology Objective: Unknown ethiology Background: Pericarditis is common in rheumatoid arthritis, mostly occurring as an extra-articular manifestation of the disease. We describe a patient with stable rheumatoid arthritis who presented with a large pericardial effusion and a compressive fibrotic pericardial mass. The patient had recently started treatment with a tumor necrosis factor-alpha (TNF-α) antagonist. Case Report: The patient was a 58-year-old woman with rheumatoid arthritis who presented with right ventricular compression caused by a pericardial fibrotic mass and a large pericardial effusion. The patient did not have active arthritis at the time of presentation. She had been started on treatment with a tumor necrosis factor-alpha (TNF-α) antagonist 4 months prior to this presentation. She was successfully treated with surgical pericardiectomy and resection of the pericardial mass. Pathologic analysis of the pericardial mass demonstrated fibrosis and no evidence of active inflammation, rheumatoid arthritis, opportunistic infection, or malignancy. Conclusions: We describe a patient with stable rheumatoid arthritis who developed subacute right heart compression syndrome secondary to pericardial effusion and fibrous pericardial mass. The exact cause of pericarditis and the pericardial mass remain uncertain. There is a need for increased awareness of the association between use of TNF-α antagonists and the possible development of an intrapericardial fibrotic mass and effusion.