For all US presidents with the same last name, which were actually related and how?

Presidents who were related:

Father and son : John Adams and John Quincy Adams

Grandfather and grandson : William H Harrison and Benjamin Harrison

Fifth Cousins : Theodore Roosevelt and Franklin D Roosevelt


http://www.arlingtonva.us/Departments/VoterRegistration/VoterRegistrationTermsTrivia.aspx

[Link added Feb. 22, 2008 by Zbeckabee]

UPDATE:

George H. W. Bush and George W. Bush (father and son)

ADDITIONALLY: List of United States Presidents by genealogical relationship has a LONG list of others though not limited to the same last name:

Jimmy Carter: ninth cousin of Warren Harding, sixth cousin of Richard Nixon

Theodore Roosevelt: third cousin three times removed of Martin Van Buren

Etc.

https://familypedia.wikia.org/wiki/Genealogical_relationships_of_Presidents_of_the_United_States