Walgreen Co. was incorporated as an Illinois corporation in 1909. The company's primary business is the operation of retail drugstores. Walgreens operates a national chain of drug stores with 5,222 locations in 45 states and Puerto Rico. In addition, the company operates 3 mail service facilities. The company serves four million customers daily and filled more than 490 million prescriptions in 2005. More than 1,500 Walgreens stores are open 24 hours.