Former president FW de Klerk announced, during his 85th birthday celebration on Thursday, that he had been diagnosed with cancer.
In a statement, De Klerk's foundation said he was diagnosed with mesothelioma, which is a cancer that affects the tissue lining of the lungs.
"Mr De Klerk will start a course of immunotherapy next week. There is no immediate threat and we are confident the treatment will be successful," said the foundation.
In October last year, De Klerk lost his son, Willem, 53, to cancer.