"I.V." is a song by Japanese heavy metal band X Japan, released on January 23, 2008 as a digital download. It is the group's first new material since "The Last Song," released almost a decade earlier in 1998. "I.V." is the main theme for the American film Saw IV, which was released on October 26, 2007.

An X Japan reunion was announced in mid-2007, with Yoshiki, Toshi, Heath and Pata all participating. In July, Yoshiki was approached about composing the main theme for Lionsgate's Saw IV (he also composed Lionsgate's Catacombs main theme), Yoshiki chose to perform the song with X Japan.

In October Lionsgate released the production notes for the movie, and according to them, X Japan would perform the main theme. This was confirmed on the 18th with an official announcement. "I.V." (written by Yoshiki) would feature Toshi, Heath, Pata and Yoshiki, along with unreleased guitar tracks by former guitarist hide, who died in 1998. According to Yoshiki, the version released is a shortened version of his original work, which was too long to fit the movie.

In regards to the meaning of the track's title "I.V.", Yoshiki originally used it as an acronym for intravenous drip, although he later noticed it would fit with the movie's title.

Originally the song was going to be released on the Saw IV soundtrack, but Yoshiki preferred to release it as an X Japan single. The song was eventually released worldwide on January 23 via iTunes Store topping the iTunes charts in Japan and being featured under "What's hot" in America. The song was released in Japan, South Korea, United States, Canada, United Kingdom, New Zealand, Australia, France, Belgium, Denmark, Germany, Spain, Finland, Greece, Ireland, Italy, Luxembourg, Netherlands, Norway, Austria, Portugal, Switzerland and Sweden.

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