Medullary carcinoma of the thyroid commonly spreads to the lymphatics and later on in the disease process, to distant sites such as lung, liver and bone. Spread to the breast is rare (only two previous reported cases) and presents a major diagnostic dilemma. We report a case of metastatic medullary carcinoma of the thyroid which presented as a primary breast nodule with ipsilateral axillary lymphadenopathy. Clearly it was important to distinguish this tumour from a breast primary as each are managed differently. Both cytological and radiological investigations were inconclusive but excision biopsy was diagnostic.