Hypertrophy and focal nodular hyperplasia with tumour-like proliferation of the C-cells was observed in thyroid tissue removed from a 64-year-old male with non-toxic adenomatous goitre and primary hyperparathyroidism. The patient was found to have an enlarged parathyroid gland consisting exclusively of waterclear cells. It appeared likely that chronic hypercalcaemia had been responsible for the C-cell proliferation. Morphological similarities between the thyroid change of the present case and the early stages of some forms of medullary thyroid carcinoma suggest that this neoplasm could arise from hyperplastic foci of C-cells. It is emphasized that chronic hypercalcaemia may be one aetiological factor in human C-cell neoplasia, including some types of medullary carcinoma. This implies that medullary carcinoma might have been secondarily induced by the parathyroid adenoma in some of those patients in whom both of these manifestations have been encountered.