Receiving a new diagnosis can feel lonely, especially when the condition is rare. For many people diagnosed with systemic mastocytosis (SM), one of the first things they learn is just how uncommon it is.
Meeting people with the same disease can make coping easier. With rare diseases, however, that kind of connection can be harder to find. This can make the condition feel even more isolating.
The question that often comes next is: How rare is it?
When a patient starts looking for that answer, they may find a confusing range of figures. So far, the clearest numbers come from European studies that counted diagnosed SM across large populations.
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What the most reliable studies show about systemic mastocytosis
The strongest evidence comes from Denmark, where researchers used nationwide registry data spanning 14 years to estimate how many people were living with SM. Because the study drew on a national health registry rather than a single hospital or self-reported cases, it captured a fuller picture than most studies on the subject. They found that 9.59 out of every 100,000 people were living with SM.
Other studies using population-based data found similar numbers. In Sweden, Ungerstedt and colleagues estimated 10.6 cases per 100,000 adults in the Stockholm region. In Italy, a study reported 10.2 cases per 100,000 adults in the Veneto region and 17.2 in Verona province.
Taken together, these numbers suggest SM affects roughly 9 to 17 per 100,000 adults — approximately one in 6,000 to 11,000.
Why the number keeps slipping
Part of the difficulty is timing. In the 2024 MASTering SM study, researchers found that people waited an average of 58.3 months (nearly five years) between their first symptoms and an SM diagnosis. Anyone still in that waiting period does not show up in a registry yet. Every prevalence estimate is, in effect, missing an unknown number of people who have the disease but do not yet have the diagnosis to prove it.
Definitions add another layer of uncertainty. Some studies count SM specifically; others count mastocytosisMastocytosis Rare disease caused by the buildup of mast cells. Cutaneous mastocytosis primarily affects the skin and is more common in children, while systemic mastocytosis affects internal organs and is more common in adults. more broadly, a category that includes other forms of mast cell diseaseMast cell disease A group of conditions in which mast cells behave abnormally by building up in excess, releasing chemicals too easily or both. Includes mastocytosis, mast cell activation syndrome and hereditary alpha-tryptasemia.. Mixing the two produces numbers that look bigger but are not measuring the same thing.
More diagnoses do not mean more disease
The 2025 Danish study found that new diagnoses of SM, particularly indolent systemic mastocytosis (ISM), have increased over time. That pattern supports the idea that more people were diagnosed because doctors became more familiar with SM and testing improved, not necessarily because more people were developing the disease.
Read more about SM testing and diagnosis
In the US, the picture is even less clear
European countries have an advantage, since centralized national health registries make population-wide tracking possible. The U.S. has no equivalent system for SM.
The closest data comes from a 2018 National Cancer Institute database study, which identified 425 adults diagnosed with SM over 14 years. This underscores how little is known about SM’s true reach in the U.S.
So, is anyone else out there?
Almost certainly, more people are living with SM than any current estimate captures. The best available research points to roughly nine to 17 per 100,000 adults, but that number likely undercounts people who are still undiagnosed, misdiagnosed or missing from the registries researchers rely on.
