The authors describe four cases of Sipple's syndrome, a combination of medullary carcinoma of the thyroid gland and pheochromocytoma, within one family. The mother, two daughters and their maternal cousin have been kept under observation. The mother died of cerebral haemorrhage, the surgical treatment applied to the sisters turned out to be successful. As a result, the integrated examination of the endocrine glands when there is a tumorous affection of one of them is of the essence. According to the authors' standpoint, under the Sipple's syndrome the surgery should be started with removing the pheochromocytoma and thereupon the special operation for the thyroid gland should be performed.