How many people get cancer of unknown primary?

The exact number of cases of cancer of unknown primary (CUP) is not known because some cancers start out being labeled as CUP, only later to have the place where they started found. Still, the American Cancer Society estimates that about 31,860 cases of CUP will be found in 2013 in the United States. This number is about 2% of all cancers. The number of CUP cases is likely to go down in the future as we create better lab tests to find out where a cancer started.