Primary malignant cardiac neoplasms are extremely rare. The occurrence of a malignant lymphoid tumour within a left atrial myxoma is highly atypical, with only one such case previously reported. Here, we describe a patient who presented with symptoms and signs of a left atrial myxoma. Subsequent specimen histology demonstrated the presence of lymphoma within the myxoma. We discuss the importance of histological diagnosis in order to best direct treatment and prognosis of such cases.