Hello! As this is my first comment on Reddit (still trying to figure it all out), I do hope you will bear with me, in case I make some form of mistake.
Thomas Jefferson never publicly acknowledged his relationship with Sally Hemings. Despite all accusations thrown at him, he remained silent, save for a letter he wrote to the secretary of the navy in his cabinet, Robert Smith, in 1805, which said,
"You will perceive that I plead guilty to one of their charges, that when young and single I offered love to a handsome lady. I acknowledge its incorrectness. It is the only one founded on truth among all their allegations against me."
His unwillingness to publicly speak out against the allegations is explained in a 1798 letter to Samuel Smith:
"Were I to undertake to answer the calumnies of the newspapers, it would be more than all my own time, & that of 20. aids could effect. For while I should be answering one, twenty new ones would be invented."
Sources:
http://www.let.rug.nl/usa/presidents/thomas-jefferson/letters-of-thomas-jefferson/jefl124.php
http://founders.archives.gov/documents/Jefferson/99-01-02-2005