Valve has revealed that Left 4 Dead has been a huge success. The PC and Xbox 360 editions of the game have combined to sell more than 2.5 million units at retail worldwide, according to the publisher.

That number does not include sales through Valve’s online distribution platform Steam, a statistic the publisher traditionally keeps under its hat. However, Valve founder Gabe Newell revealed in February that during a one-weekend-only half-off sale through Steam, Left 4 Dead sales skyrocketed 3,000 percent.