The authors report on the case of a 5,8 year-old girl whose father died of medullary thyroid carcinoma. When she was 4,5 year-old, her physical examination was normal but plasma calcitonin and katacalcin (PDN-21) levels were abnormally high in response to pentagastrin infusion. Total thyroidectomy was performed and immunohistochemical staining showed confluent C-cell hyperplasia. No recurrence occurred in this patient over four years on follow-up.