That was an increase of five from the last time the Pentagon released results of its reviews, which were completed in January but required additional time for final approval.
Most of the people held at Guantanamo are Afghans, Pakistanis and others captured after the US invasion of Afghanistan in October 2001.
They were labeled enemy combatants, which the Bush administration decided did not obligate the United States to give them prisoner-of-war status under the Geneva Conventions.
Navy Secretary Gordon England, overseer of this and a related review of the detainee cases, told a Pentagon news conference that five of the 38 have been sent to their home countries. The 33 others remain at the Guantanamo base awaiting transportation.
In all, 558 cases were reviewed, although presently only 540 detainees are at Guantanamo Bay; some were released for other reasons during the six months required to complete the reviews.
Some have been there for more than three years; criminal charges have been brought against only four.
The Pentagon defines enemy combatant as "an individual who was part of or supporting the Taliban or al-Qaida forces, or associated forces that are engaged in hostilities against the United States or its coalition partners."
The category includes "any person who committed a belligerent act or has directly supported hostilities in aid of enemy armed forces."
England did not provide a breakdown of the nationalities of the 38, saying he was unsure whether US policy would permit that. The State Department is arranging their homeward trips after more than three years in captivity.