College students typically receive marketing offers in the mail from upwards of a hundred companies each year.

But the flood of mail masks a surprising reality--only two companies are responsible for most of the unsolicited marketing received by undergraduates at Harvard and other colleges across the country.

Educational List Sources (ELS) and American Student Lists (ASL) both compile vast databases of information about college students in the U.S. and rent them to companies interested in marketing their products on college campuses.

The information that ELS and ASL provide--including students' names, addresses, telephone numbers and e-mail address--allows companies to tailor their pitches to a pool of potential consumers that are flush with disposable income.

The service is big business.

With about 3,700 customers and annual revenues of $20 million, ASL has by far the largest presence in the student mailing market. In documents filed with the Securities and Exchange Commission (SEC), ASL reports that it maintains a database of 30 million names and addresses, in categories ranging from "College" to "pre-school."

The Crimson contacted a series of companies that have an extensive direct-mail marketing presence on college campuses and inquired about the source of their mailing lists. All of the companies, with the exception of American Express, which would not discuss the source of its direct-mailing lists, said they used either ASL or ELS. Several companies used both list providers.

How these list providers compile their information and how they use it is part of a complicated and sometimes cloudy relationship that touches most every college campus in the U.S.