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The exact number of cancers of unknown primary (CUP) diagnosed each year is unknown, because some cancers start out being diagnosed as unknown primary, but the primary site is found later. Still, the American Cancer Society estimates that about 34,170 cases of cancer of unknown primary will be diagnosed in 2016 in the United States. This number represents about 2% of all cancers. As more sophisticated lab tests become available to determine where a cancer started, the number of cancers of unknown primary may go down.

Visit the American Cancer Society’s Cancer Statistics Center for more key statistics.


Last Medical Review: 07/02/2014
Last Revised: 01/27/2016