High red blood cells and low mean red blood cell volume means that the number of red blood cells in the peripheral blood is above the normal range, but the mean volume of red blood cells is below the normal range. This condition is commonly seen in a disease called true erythrocytosis, which is a myeloproliferative disorder. The number of red blood cells in the peripheral blood can be abnormally high due to the disorderly proliferation of progenitor cells in the red lineage, and the number of red blood cells in the peripheral blood is significantly high. However, due to the rapid proliferation of red blood cells, the concentration of iron in the body is insufficient, and a low red blood cell volume often occurs. There can also be a low mean hemoglobin volume and a high red blood cell count on routine blood tests, but a low mean red blood cell volume, which can occur in patients with true erythrocytosis. The high red blood cell count and low red blood cell volume improve significantly when the patient is given hydroxyurea therapy or given bloodletting therapy and given iron supplements.