Hairy cell leukemia

ICD-O-1 Morphology

9941/3: Leukemic reticuloendotheliosis
Effective 1978 - 1991

ICD-O-2 Morphology

9941/3: Leukemic reticuloendotheliosis
Effective 1992 - 2000

ICD-O-3 Morphology

9940/3: Hairy cell leukemia
Effective 2001 and later


for cases diagnosed 1978 and later

Primary Site(s)

Primary site must be bone marrow (C421)

Abstractor Notes

Do NOT use this code for hairy cell leukemia variant (See 9591/3). When HCL is diagnosed in people in their 20's, it is hairy cell leukemia variant (9591/3)

The diagnosis is best made on bone marrow. HCL infiltrates are found in the red pulp of the spleen. The white pulp is typically atrophic. The liver may show infiltrates of hairy cells, predominantly in the sinusoids. Lymph node infiltration may occur especially in advanced disease.

Treatment options will depend on the following: Number of hairy cells vs healthy blood cells in the blood and bone marrow, swollen spleen, presence of infection or whether the leukemia has recurred (come back) after previous treatment. Treatment often results in long-lasting remission

Hairy Cell leukemia has the following groups:

1. Untreated Hairy Cell Leukemia: newly diagnosed

2. Progressive Hairy Cell Leukemia: previous treatment with either chemotherapy or splenectomy and the following occur: increase in number of hairy cells in the blood or bone marrow OR number of red blood cells, white blood cells or platelets is lower than normal

3. Relapsed Hairy Cell Leukemia: Leukemia has come back after treatment

4. Refractory Hairy Cell Leukemia: Nonresponsive to treatment

Other therapy includes watchful waiting.

Diagnostic Confirmation

This histology can be determined by positive histology (including peripheral blood) with or without genetics and/or immunophenotyping. Review the Definitive Diagnostic Methods, Immunophenotyping and Genetics Data sections below, and the instructions in the Hematopoietic Manual for further guidance on assigning Diagnostic confirmation.


Not Applicable

Module Rule


Alternate Names

Leukemic reticuloendotheliosis [OBS]


Hairy cell leukemia (HCL) is a cytologically and immunophenotypically distinct, indolent neoplasm of small mature lymphoid cells with oval nuclei and abundant cytoplasm with so-called hairy projections involving peripheral blood and diffusely infiltrating the bone marrow and splenic red pulp.

Definitive Diagnostic Methods

Bone marrow biopsy
Genetic testing

Genetics Data

Coexpression of multiple clonally related Ig-isotypes
BRAF V600E mutations (except hairy cell variant)
IGHV genes with somatic hypermutation


Annexin Ai (ANXA1)+ (expression/positive)
CD5- (no expression/lacking/negative)
CD10- (no expression/lacking/negative)
CD11c+ (expression/positive)
CD20+ (expression/positive)
CD22+ (expression/positive)
CD25+ (expression/positive)
CD103+ (expression/positive)
CD123+ (expression/positive)
CD200+ (expression/positive)
Cyclin D1+ (expression/positive)
DBA.44+ (expression/positive)
FMC7+ (expression/positive)
TBX21 (TBET)+ (expression/positive)


Other therapy

Transformations from


Corresponding ICD-9 Codes

202.4 Leukemic reticuloendotheliosis

Corresponding ICD-10 Codes

C91.4 Hairy-cell leukemia

Corresponding ICD-10-CM Codes (U.S. only)

C91.4 Hairy-cell leukemia (effective October 01, 2015)

Signs and Symptoms

Easy bruising or bleeding
Fever or frequent infections
Pain or a feeling of fullness below the ribs
Painless lumps in the neck, underarm, stomach or groin
Shortness of breath
Weakness or feeling tired
Weight loss for no known reason

Diagnostic Exams

Blood chemistry studies
CT (CAT) scan
Cytogenetic analysis
Flow cytometry
Peripheral blood smear

Progression and Transformation


Epidemiology and Mortality

Age: 50 years median age
Incidence: 2% of lympohid leukemias
Sex: male predominance
Survival: 10 years survival rate >90%


Swerdlow SH, Campo E, Harris NL, Jaffe ES, Pileri SA, Stein H, Thiele J (Eds):
WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues (Revised 4th edition)
IARC: Lyon 2017
Section: Mature B-cell neoplasms
Pages: 226-228

International Classification of Diseases for Oncology, Third Edition, Second Revision. Geneva: World Health Organization, 2020.
Section: ICD-O-3.2 (2020) Morphological Codes

National Cancer Institute
Section: General Information About Hairy Cell Leukemia