Erythrocytosis is an increase in the level of red blood cells in the blood. May be physiological or pathological. It accompanies diseases of the bronchopulmonary system, heart defects, kidney diseases, hormonally active neoplasms, high blood pressure, significant fluid loss. Manifestations vary, moderate hepatosplenomegaly, red cyanosis of the skin, varicose veins, sometimes bleeding, thrombosis are possible. Erythrocytosis is determined by the results of a clinical blood test. Treatment includes therapy of the underlying pathology, lifestyle correction, diet, bloodletting.