In anticipation of bulk sales, the Half-Life developers are launching a $25m advertising campaign for the sequel.
Valve president Gabe Newell said: "Based on the strength of pre-orders, Left 4 Dead 2 will be the fastest-selling product in Valve's history. Left 4 Dead 2 has consistently run at 300% of Left 4 Dead's numbers."
Doug Lombardi, Valve's vice president of marketing, added: "Left 4 Dead 2 has already set the record for greatest number of pre-orders in our company's history, and we're still over a month out from shipping."
Left 4 Dead 2 will be released on November 17 on Xbox 360 and PC.