Introduction Mediastinal tumours are often seen in patients aged 30-50 years. Most of them are neurogenic tumours and thymomas. Others include lymphomas, phaeochromocytomas, melanomas, germ cell tumours and thyroid and parathyroid lesions. Although primary cardiac tumours are rare, metastasis in the heart is commoner than primary cardiac tumours. Case report A 41 year old woman was admitted to the Teaching Hospital, Karapitiya with a recent onset progressive dyspnoea. She was found to have a rapidly filling pericardial effusion. She died on the third day of admission. A whitish mass infiltrating the full thickness of the right atrial myocardium was found at the autopsy. A similar tumour was present in the anterior mediastinum measuring 7x3cm. There was no direct connection between the two tumours. H&E stained sections revealed a similar microscopic appearance in both mediastinal and cardiac masses. The differential diagnosis included lymphoma, melanoma, carcinoma and germ cell tumors. Immunohistochemical staining for LCA, pan cytokeratin, S100 and PLAP were done and only S100 was positive. Conclusion Negative staining for LCA, pan cytokeratin and PLAP excluded the possibility of lymphoma, carcinoma and germ cell tumours. Positive staining for S100 confirmed the diagnosis of melanoma. This was diagnosed as a rare case of primary mediastinal malignant melanoma with a right atrial metastasis causing pericardial effusion.

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