Fan fiction, stories by amateur writers about characters from their favorite books, movies and television shows, was once mainly a fringe pursuit. Now, it's changing the world of fiction, as Internet exposure helps unknown authors find mainstream success. Some Web sites are attracting unprecedented numbers of readers and, in some cases, leading to book deals. They are also feeding the appetites of readers and viewers who can't get enough of shows like "Lost" or "House."

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One sign of the growing influence of these authors and stories is that media companies, usually quick to go after people who use their copyrighted material, are increasingly leaving fan fiction writers alone. Mindful of the large, loyal audience the writers represent, many companies are adopting an attitude one media professor describes as "benign neglect." While most professional writers say their lawyers advise them not to read fan fiction to protect themselves against charges of plagiarism, some say they check the numbers of fan fiction stories posted about their work regularly as a measure of their success.

The rise of fan fiction is part of the spread of amateur-created content online, from viral videos to music playlists and blogs. Increasingly, audiences have become used to watching videos posted by other users on sites such as YouTube and MySpace. Reading fiction online is another extention of this trend.

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Some authors take a less friendly view of the genre, however. "No matter how flattering, it's still robbery," says fantasy novelist Chelsea Quinn Yarbro, whose vampire works have inspired a number of fan-fiction writers. She estimates that her attorney has sent out about 20 "cease and desist" letters to writers and owners of fan sites. Ms. Yarbro says this has caused some of the writers and sites to take their stories down.