The I.R.S. announced on Wednesday that it had revoked the tax exemptions of 275,000 nonprofit organizations after they did not meet legal requirements to file annual tax forms.

The action shrinks the nation’s growing nonprofit sector by roughly 17 percent, to about 1.3 million charities, trade associations, membership groups and labor unions.

Lois Lerner, director of the division of the Internal Revenue Service that oversees tax-exempt groups, said the agency believed most of the organizations on the list were defunct, though there was really no way to know because so many of them simply could not be reached.

“In many cases, we didn’t have a good address because the last one was many years old and they hadn’t had to file since then because they weren’t big enough,” Ms. Lerner said.