During most of the 20th century, U.S. copyright law required at least one copy of every American film to be deposited at the
Library of Congress, at the time of
copyright registration, but the
Librarian of Congress was not required to retain those copies: "Under the provisions of the act of March 4, 1909, authority is granted for the return to the claimant of copyright of such copyright deposits as are not required by the Library."
 Of American
silent films, far more have been lost than have survived, and of American
sound films made from 1927 to 1950, perhaps half have been lost.
The phrase "lost film" can also be used in a literal sense for instances where footage of
deleted scenes, unedited, and alternative versions of feature films are known to have been created, but can no longer be accounted for. Sometimes, a copy of a lost film is rediscovered. A film that has not been recovered in its entirety is called a partially lost film. For example, the 1922 film
Sherlock Holmes was eventually discovered, but some of the footage is still missing.