Valve has announced that the first downloadable add-on for Left 4 Dead 2 will be called The Passing and is due out in spring 2010 for PC and Xbox 360.
Pricing will be announced "closer to launch".
The Passing is set in rural Georgia just after L4D2's Dead Center campaign and brings the current team of survivors together with the foursome from the first game.
Valve has said players will control the L4D2 team, but the press release doesn't rule out a handover at some point.
The Passing will consist of a new co-operative campaign, which will also introduce new Survival, Scavenge and Versus arenas.
On top of that, there will be a new gun, a new melee weapon and a new uncommon common infected.
Valve's Doug Lombardi has said The Passing will be "a huge offering of new gameplay content, with something new for every game mode plus a new uncommon common and weaponry".