Syndrome of multiple endocrine neoplasia, type IIa, is a rare disturbance made by medullar carcinoma of the thyroid gland, pheochromacytoma and hyperparathyroidism. The article deals with three patients with multiple endocrine neoplasia, type IIa, with special emphasis to detection of the disease in time and treatment of the patients in the early phase of the illness. This is a hereditary autosomal dominant disturbance. Tests performed among members of family with high risk factors of the disease are of great importance. In this way early diagnosis is possible, and consequently, prevention of fatal issue due to pheochromocytoma and medullar carcinoma.