Night Sweats
What Are "B Symptoms" (Night Sweats, Fever, Weight Loss)?
"B symptoms" is a clinical shorthand for a specific trio of symptoms that clinicians look for when staging certain lymphomas: unexplained fever, drenching night sweats, and unexplained weight loss of more than about ten percent of body weight over roughly six months. The name comes from the staging system for Hodgkin lymphoma and is used in a small number of related contexts. It does not mean that any one of these symptoms, on its own, points to cancer.
If you have looked up night sweats online, you have probably seen the phrase "B symptoms" and felt a jolt of worry. The term has a specific meaning and a specific purpose, and knowing what it actually refers to is calmer than the impression an internet search often leaves.
This entry explains where the term comes from, what it includes, and how clinicians use it in practice.
Where the term comes from
"B symptoms" comes from the Ann Arbor staging system, developed in the 1970s for Hodgkin lymphoma and later extended to non-Hodgkin lymphomas. In that system, each stage carries an "A" or "B" letter: "A" if the specific trio of systemic symptoms is absent, "B" if it is present. Their presence can matter for how a lymphoma is described and how it is approached.
Because the term is precise, clinicians use it deliberately. It is not a general phrase for feeling unwell; it refers to a defined set of symptoms in a defined clinical context.
What the trio actually is
The B symptoms are: unexplained fever above about 38 degrees Celsius (100.4 Fahrenheit) that recurs without a clear cause, drenching night sweats that soak the bedclothes and require changing them, and unexplained weight loss of more than ten percent of body weight over roughly six months.
The key word running through the definition is unexplained. A fever tied to a known infection, night sweats after a warm night or a couple of drinks, or weight loss during a diet or after a stomach illness do not fit the picture. What clinicians look for is symptoms with no obvious ordinary cause, appearing together, and persisting.
Why one symptom alone is not a diagnosis
Night sweats, on their own, are common and have many benign causes: warm bedding, alcohol, spicy food, anxiety, hormonal shifts, and certain medications. Isolated night sweats are not a B symptom in the technical sense, and they are not, by themselves, a sign of lymphoma or any other specific disease.
The reason clinicians pay attention to the trio together is that the combination is less common and less easily explained by everyday causes. Even then, the presence of B symptoms is a prompt for careful assessment, not an announcement of a diagnosis: many people who report the full trio turn out to have something benign, and many people with lymphoma do not have B symptoms at all.
How the term is used in practice
In clinics, the phrase mostly comes up in three places: when a clinician is taking a careful history from someone with unexplained symptoms, when a lymphoma is being staged, and when treatment response is being tracked over time. Outside those settings, the term travels around health sites where it can sound more alarming than it is meant to.
The calm, accurate use of the term is as a checklist: fever, drenching sweats, and unexplained weight loss, together and without another explanation, are the pattern that a clinician wants to hear about. Any one of them alone, or any of them with a clear ordinary cause, is a different conversation.
Frequently asked questions
Are night sweats always a B symptom?
No. Only drenching night sweats that soak the bedclothes and require changing them, appearing together with unexplained fever and unexplained weight loss, meet the B-symptom definition. Ordinary sweating from a warm room, alcohol, hormones, or medications does not fit.
What temperature counts as a B-symptom fever?
Clinicians generally use unexplained fever above about 38 degrees Celsius (100.4 Fahrenheit), recurring without a clear cause. A one-off fever during a cold or flu is not what the definition refers to.
How much weight loss is significant?
The definition uses unexplained weight loss of more than ten percent of body weight over roughly six months. Weight loss during intentional dieting, after an illness, or with a known cause does not meet the definition.
Do I have lymphoma if I have night sweats and lost weight?
Not necessarily. Even the full trio prompts assessment rather than a diagnosis, and many people with these symptoms have something benign. The point of naming B symptoms is to help clinicians decide what to look at, not to draw conclusions from a search bar. If the pattern fits, that is a conversation to have with a clinician.
Sources & further reading
The reputable organizations our editorial team draws on for the anatomy, definitions, and safety guidance behind this page, and where you can read more on each topic.
General educational information about sweating. Not medical advice, and not a substitute for diagnosis or treatment by a qualified healthcare professional.