Occasional morphologically biased naïve cells are small numbers of immature cells observed in a blood test. Immature cells are morphologically naïve cells and refer to cells that have not yet developed. The development of blood cells of all lineages is a continuous process of transformation from immature to mature blood cells, but in normal peripheral blood, there should be no naïve cells and the percentage of rod-shaped cells is less than 5%. If the percentage of naïve cells is higher than 5%, it may indicate infectious diseases such as viral infections, and malignant hematologic diseases such as leukemia. Also, naïve cells are present after tumor chemotherapy. If the percentage of morphologically biased naïve cells in the peripheral blood is high or elevated without a cause, appropriate tests, such as bone marrow smear, should be performed to clarify the cause of the disease under the guidance of a physician and prompt treatment.