The early diagnosis of medullary thyroid carcinoma was made in two among six examined siblings belonging to two sibships that were offsprings of multiple endocrine neoplasia type II parents. The calcitonin secretory reserve was determined by a combined test using Ca++ (2 mg/kg) and pentagastrin (0.5 mcg/kg), intravenously. Two abnormal tests made on different days supported the diagnosis. Basal calcitonin levels were moderately high (90-500 pg/ml; NL = 15-85 pg/ml) and peak levels were also abnormal (480-1500 pg/ml; NL less than 320 pg/ml), in both cases. Total thyroidectomy associated to prophylactic resection of lymph nodes from central neck region were performed in both. A small nodule (3-5 mm) was found in each lobe in both cases. Pathological and immunocytochemical data supported the diagnosis of medullary thyroid carcinoma. C-cell hyperplasia was present in the peritumoral zones. Pheochromocytoma and definite hyperparathyroidism were not detected in these cases. Two years after surgery, basal and stimulated serum calcitonin levels remained normal. Carcinoembryonic antigen levels were and continue to be normal, in both. These seem to be the first cases published in this country in which this early diagnosis was made.