Abstract Background/Aims IgG4-related sclerosing disease is a systemic disease characterized by extensive IgG4-positive plasma cells and T-lymphocyte infiltration of various organs and tumour-like involvement of one or more exocrine glands or other extranodal sites. The major involved organs are the salivary glands, lacrimal glands, pancreas and retroperitoneal space. Cardiac masses in IgG4 disease include hematoma, tumours, vegetations, calcific lesions and other rare conditions. Cardiac tumours are rare and have a variety of benign and malignant entities. We herein report the case of a patient with antiphospholipid antibody syndrome with a mass lesion in the right atrium, histologically proven as IgG4-related disease. Methods A 52-year-old female, a known case of primary antiphospholipid antibody syndrome for 14 years, with five first trimester abortions and positive lupus anticoagulant, on anticoagulation with vitamin K antagonist, presented with: shortness of breath, bilateral lower limb swelling and exertional dyspnoea- grade 3, since one week before admission, associated with orthopnoea, without any h/o decreased urine output or facial puffiness. On examination, she had hypertension with raised JVP and loud P2 possibilities of myocarditis, hypertensive heart failure, and severe PAH with CCF were considered. Investigations showed iron deficiency anaemia, normal complements, and negative anti-dsDNA antibodies. she was treated with diuretics, PDE inhibitors and antihypertensives, 2D echo showed haziness in the right atrium, possible thrombus (1.5 × 2.3 cm), CTPA - showed enhancing mass lesion in right atrium wall, neoplastic origin was considered. Results To further evaluate the mass in the right atrium, a PET scan was performed, which showed a metabolically active isolated mass lesion in the right atrium, Cardiac MRI showed an ill-defined lobulated T1 hypointense, T2/STIR isointense lesion involving the right atrial wall, possible lymphoma/sarcoma. Planned for excision/debulking via sternotomy approach, intraoperative trans oesophageal echo showed right atrial mass of 25 cms possibility of lymphoma/sarcoma, during surgery mass could not be excised but a biopsy was taken and sent for histopathological analysis. The report was suggestive of IgG4 disease, plasma cells stain positive for IgG4 with ratio of IgG4 to IgG of 0.4. IHC results showed no evidence of malignancy. Serum IgG4 levels was 145mg/dl (<135mg/dl). Diagnosis of IgG4 disease was made and the patient was started on steroids, repeat 2D echo after 6 months showed that the mass in the right atrium had resolved and the patient was asymptomatic. Conclusion In conclusion, this was a rare case of primary antiphospholipid antibody syndrome, presenting as a cardiac mass which was difficult to distinguish from malignant lymphoma by a pathological examination alone, hence a biopsy was done to analyze the clonality of the cardiac tumour and accurate diagnosis of IgG4-RD was made. Disclosure K. Yerram: None. P. Devarasetti: None. L. Rajasekhar: None.