A December 7, 2010 U.S. Census Bureau press release notes that:
"In 2007, women owned 7.8 million businesses and accounted for 28.7 percent of all businesses nationwide, according to the U.S. Census Bureau's Survey of Business Owners. These firms generated $1.2 trillion in receipts, about 3.9 percent of all business receipts nationwide. (See Table 1 [Excel].)
Businesses owned equally by men and women numbered 4.6 million firms (17.0 percent of all businesses) and generated $1.3 trillion in receipts (4.2 percent of all receipts)."
See also: Women-Owned Businesses (U.S. Census blog)