A case of melanin-producing medullary carcinoma of the thyroid gland is reported. The histological characteristics of the tumour were typical of medullary carcinoma, immunohistochemical testing for calcitonin was positive, and neurosecretory granules were found to be present on ultrastructural examination. In addition the tumour was producing melanin and melanosomes were identified in the same cells which contained neurosecretory granules, suggesting that the tumour arose from a common precursor cell of neural crest origin capable of producing both calcitonin and melanin.