Merck Manual

Please confirm that you are a health care professional

honeypot link

Overview of Leukemia


Ashkan Emadi

, MD, PhD, University of Maryland;

Jennie York Law

, MD, University of Maryland, School of Medicine

Reviewed/Revised Oct 2023 | Modified Nov 2023
Topic Resources

Leukemia is a malignant condition involving the excess production of immature or abnormal leukocytes, which eventually suppresses the production of normal blood cells and results in symptoms related to cytopenias.

Malignant transformation usually occurs at the pluripotent stem cell level, although it sometimes involves a committed stem cell with more limited capacity for self-renewal. Abnormal proliferation, clonal expansion, aberrant differentiation, and diminished apoptosis (programmed cell death) lead to replacement of normal blood elements with malignant cells.

The American Cancer Society estimates that in the United States in 2023 there will be about 60,000 new cases of leukemia (of all types) in adults and children and about 24,000 deaths.

Classification of Leukemia

The current approach to classifying leukemia is based on the 2016 World Health Organization (WHO) system (classification for hematopoietic neoplasms). The WHO classification is based on a combination of clinical features and morphology, immunophenotype, and genetic factors. Other less commonly used classification systems include the French-American-British (FAB) system, which is based on the morphology of the abnormal leukocytes.

Leukemias are commonly also categorized as

  • Acute or chronic: Based on the percentage of blasts or leukemia cells in bone marrow or blood

  • Myeloid or lymphoid: Based on the predominant lineage of the malignant cells

The four most common leukemias and their distinguishing features are summarized in the table .


Acute leukemias

Chronic leukemias

Myelodysplastic syndromes

Myelodysplastic syndromes Myelodysplastic Syndrome (MDS) The myelodysplastic syndrome (MDS) is group of clonal hematopoietic stem cell disorders typified by peripheral cytopenia, dysplastic hematopoietic progenitors, a hypercellular or hypocellular... read more are a group of clonal hematopoietic stem cell disorders unified by the presence of distinct mutations of hematopoietic stem cells. They involve progressive bone marrow failure but with an insufficient proportion of blast cells (< 20%) for making a definite diagnosis of acute myeloid leukemia; 40 to 60% of cases evolve into acute myeloid leukemia.

Leukemoid reaction

A leukemoid reaction is a neutrophil count > 50,000/mcL (> 50 × 109/L) not caused by malignant transformation of a hematopoietic stem cell. It can result from a variety of causes, particularly other cancers or systemic infection. Usually the cause is apparent, but apparent benign neutrophilia can be mimicked by chronic neutrophilic leukemia or chronic myeloid leukemia.

General reference

Risk Factors for Leukemia

Risk of developing leukemia is increased in patients with

More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

Drugs Mentioned In This Article

Drug Name Select Trade
DROXIA, HYDREA, Mylocel, Siklos
Alkeran, Evomela
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
quiz link

Test your knowledge

Take a Quiz!