The percentage of U.S. companies that monitor their employees' internal e-mail has almost doubled since 1997. The American Management Assn. International (AMA) says that 27% of the companies it surveyed in April report keeping tabs on e-mail, compared with 15% in 1997. The major reason: e-mail is increasingly being used as evidence in employee lawsuits. "The Microsoft antitrust case has shown how e-mail creates an electronic paper trail that can be used by both sides in a legal dispute," says Eric Greenberg, director of management studies at the AMA. In total, the percentage of companies that use electronic surveillance to monitor employee activities jumped to 45% compared to 35% two years ago. The second most-often-used form of corporate surveillance: storage and review of computer files. Companies said the surveillance is done to discourage employees from surfing the Internet and wasting company resources on non-business activities.