The Letheans are a telepathic species, whose telepathic attacks are almost always fatal. Their skills led them to being hired by various criminal groups in the 24th century. They are known for being incredibly stubborn.
In 2371, a Lethean named Altovar attempted to acquire a few hundred milliliters of bio-mimetic gel from Julian Bashir. After Bashir refused to give him the gel, Altovar telepathically attacked him, placing him in a coma. Altovar was later arrested by Odo. (DS9: "Distant Voices")
A year later, Toral, the illegitimate son of Duras, hired another Lethean, Soto, to read Kor's mind, after the Dahar Master discovered the location of the mythical Sword of Kahless. (DS9: "The Sword of Kahless")
- Letheans figure prominently in the PC game, Klingon Honor Guard, as one of the more formidable adversaries faced by the player, in the role of an Honor Guard trainee. In the game, the Letheans are always at least partially cloaked, and only become visible to the player at the moment they attack, or when they are killed or wounded. The Letheans attack using their telepathic abilities which are illustrated during game play by sparks of lightning-like energy, or, during an intense attack; a flash resembling ball lightning. In the game play manual, the name of the species is spelled "Lethians".
- Letheans are one of the default playable species in Star Trek Online as part of the Klingon Empire faction. 
- Michael Westmore discusses the creation of the Lethean makeup in the "Michael Westmore's Aliens, Season Four" special feature on the DS9 Season 4 DVD.
- The name is derived from the mythological (Greek) river "Lethe" that runs through Hades and has the ablility to cause amnesia and forgetfullness to any that drank from it. It also is the name of a spirit of forgetfullness and in the classical Greek it translates to "Oblivion".(citation needed • edit)