How Common Is CMML, and Who Develops This Blood Cancer?

Chronic myelomonocytic leukemia (CMML) is a rare blood cancer that primarily affects older adults. In the United States, less than one person out of every 100,000 is diagnosed with it each year ^[1]. While demographic patterns show it is most often recorded in older men and people classified as white in registries, these statistics describe populations rather than predicting who can get the disease ^[1][2][3].

What these numbers mean for you: An incidence rate is a population average, not your personal chance of developing a disease or a prediction of your outcome. Demographics do not by themselves determine your risk or your treatment success. Instead, clinicians look at specific features of your disease—such as your blood counts, the percentage of blast cells in your bone marrow, and the results of genetic testing (for mutations in genes like ASXL1, TET2, or RUNX1)—to understand your prognosis and guide your care ^[4][5][6].

Incidence and Prevalence

Incidence refers to the number of new cases diagnosed each year. Recent data from the United States estimates an age-adjusted incidence of about 0.63 new cases per 100,000 people annually, measured from 2001 to 2019 ^[1]. Earlier studies looking at registries in the U.S. and Switzerland from 1999 to 2014 found stable age-standardized rates of 0.32 to 0.38 cases per 100,000 person-years ^[7]. Because these estimates come from different registries, time periods, and case definitions, they should be viewed cautiously as separate findings rather than proof of a single trend.

Prevalence refers to the total number of people living with the disease at a given time. For CMML, a reliable population-based prevalence is difficult to establish ^[8]. These statistics are also sensitive to how the disease is diagnosed ^[9]. For example, separate diagnostic updates in 2022 by the World Health Organization (WHO) and the International Consensus Classification (ICC) altered the criteria for diagnosing CMML ^[9]. Because of these changes, certain borderline cases that were previously classified as myelodysplastic syndrome (MDS) might now be classified as CMML if they meet specific criteria like sustained monocytosis ^[9].

Age and Sex

CMML is most often diagnosed in older adults, although it can occur in younger adults as well ^[1].

• Age: The median age at diagnosis in a recent national analysis was 75 years ^[1]. In large U.S. registries, more than half of patients are diagnosed between the ages of 60 and 79 ^[2].
• Sex: Men are diagnosed more frequently than women, representing approximately 62% of reported cases ^[1][2].

Racial and Geographic Distribution

When reviewing registry statistics, it is important to distinguish between the proportion of cases (the makeup of everyone in a database) and incidence rates (how often a disease occurs within a specific population). In U.S. databases, patients classified as white make up the vast majority (roughly 79% to 90%) of recorded CMML cases ^[1][2]. A separate analysis noted that incidence rates in the U.S. were highest among white populations, while being lower among women, Black people, and Asian or Pacific Islander populations ^[3].

However, these classifications do not establish race as a biological cause for the disease ^[3]. Differences in diagnostic practices, healthcare access, and the way registries categorize race and Hispanic ethnicity can all affect who is officially counted ^[10][9]. Detailed geographic comparisons are largely limited to data from the U.S. and Switzerland, making a global picture incomplete ^[7].

Causes and Risk Factors

Most people who develop CMML have no identifiable, preventable cause for the disease. It typically arises from acquired changes in blood-forming cells over a person’s lifetime. However, research has identified a few specific risk factors:

• Prior Treatments: In retrospective cohorts, approximately 7% to 11% of CMML cases are “therapy-related” (t-CMML), meaning they developed after the patient received chemotherapy or radiation for a different condition ^[11][12][13]. When this occurred, the median time from the prior treatment to developing CMML was roughly 6.5 to 7 years ^[14][12].
• Inherited Predispositions: A small subset of cases is linked to rare inherited genetic conditions, such as those involving mutations in the DDX41 or RUNX1 genes ^[15][16][17]. Because these inherited causes are uncommon, genetic counseling and testing are typically only considered if there is a strong family history of blood cancers or if the disease occurs at an unusually young age ^[16].
• Other Associations: Some studies have noted associations between CMML and a history of smoking, chronic inflammation, or autoimmune disease ^[18]. However, these are observational links and do not prove that these factors directly cause CMML ^[18].