"Dr. Guido Westerwelle, founder of the Westerwelle Foundation and former Minister for Foreign Affairs of Germany, has been diagnosed with acute leukaemia," said Alexander Vogel, manager of the Westerwelle Foundation.
"Guido Westerwelle is undergoing medical treatment in order to fully recover," Vogel said.
Further details were not given.
Leukemia is a cancer that begins in blood-forming tissue, such as the bone marrow, and causes large numbers of abnormal blood cells to be produced and enter the bloodstream.
Westerwelle was foreign minister from 2009 until the end of 2013 under the last government, in which the FDP stood as junior coalition partners to Chancellor Angela Merkel's conservative Christian Democrats (CDU).
Westwelle stepped down as vice chancellor and FDP chairman in 2011.