A leukocyte count of 3.5 x 10^9/L is a mild leukopenia, and whether or not a leukocyte count of 3.5 x 10^9/L is severe is closely related to the etiology of the cause of the mild leukopenia. In clinical practice, there is a patient with a long-term low white blood cell count, but the patient does not have any symptoms, nor fever or fatigue. This condition is probably benign leukopenia, also known as inert leukopenia, which is only caused by an abnormal distribution of white blood cell count, and the true number and function of white blood cells are not affected and have no effect on the body. However, if a patient has a leukocyte count of 3.5×10^9/L, along with a decrease in hemoglobin and a decrease in platelets, it is likely to be caused by a blood system disorder, the most common of which include aplastic anemia, myelodysplastic syndrome, and even some acute leukemias can have a leukocyte count of 3.5×10^9/L, and the consequences of these disorders are very serious. So whether leukocyte 3.5×10^9/L is serious depends critically on the specific disease causing the leukopenia.