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Risks and causes

Cancer of unknown primary (CUP) is not one single disease. There are more than 200 different types of primary cancer. So doctors can't be specific about the possible causes or risk factors of CUP, because they don't know which type of cancer it is.

General causes of cancer

Different types of cancer have different causes. For example, the most common cause of lung cancer is smoking. Smoking is also a risk for many other cancer types such as head and neck, bladder and cervical cancer. Overexposure to ultraviolet light (from the sun or sun beds) is the main cause of skin cancer.

Generally speaking, cancer is a disease caused by many different factors coming together, rather than one single thing. Our risk of cancer depends on a combination of our genes, our environment and our lifestyle.

How you may feel

People diagnosed with cancer often feel strongly that they must find out what caused it. But for many types of cancer there is no obvious cause.

Cancer can just happen because of bad luck. Over years, a cell gathers changes to its genetic code (DNA) that eventually make it become cancerous.

Cancer causing chemicals, such as those in cigarettes, may trigger one or two of the changes. But often, something just goes wrong in the DNA of the cell by itself – a bit like a spelling mistake in the genetic code. The damaged cell then begins to grow and divide much more quickly than normal cells. Or it doesn't die off as a normal cell would. The overgrowth of cells forms a tumour.

Being diagnosed with any type of cancer is difficult to accept. You may find yourself spending a lot of energy trying to work out what caused it.

Remember that often the cause of the cancer is never found. And if doctors do find the cause, this is unlikely to change your treatment or how well it works. 


Everyone copes with this in their own way. You may find it helpful to focus your energy on getting through your treatment and staying as well as you can.

If you have questions about cancer of unknown primary, you can talk to Cancer Research UK's information nurses. You can call them on freephone 0808 800 4040, from 9am to 5pm, Monday to Friday.
Last reviewed: 
21 May 2020
  • Cancer and its Management (7th edition)
    J Tobias and D Hochhauser
    Wiley-Blackwell, 2015

  • Overview of the classification and management of cancers of unknown primary site

    J Hainsworth and others ​

    UpToDate, 2017

  • Cancer Principles & Practice of Oncology (10th edition)

    V T DeVita and others

    Wolters Kluwer, 2015