Connections

Mentioned in the DVD commentary.

Kakihara appears in this film.

pre-quel

Poster for Koroshiya 1 (2001) appears in a scene of Last Life in the Universe (2003)

The scene where a crazy 88 is vertically sliced in half.

Ichi's shoe-blade references Rosa-Klebb's from this James Bond movie.

Both films feature a Yakuza reaching into their pocket, causing others to cautiously reach for their own guns, only for the Yakuza to pull out their Yakuza family pin and place it on a table.

Alex and Ichi are both mentally controlled against their will, while Alex can't commit violent actions, Ichi is forced to do them

Jiro, Saburo, and Ichiro are named for characters from this anime.

Sardu (from "Bloodsucking Freaks", a.k.a. "The Incredible Torture Show") and Kakihara (from "Ichi The Killer") have a similar reaction when they are getting close to death.