The personal details of up to 143 million US consumers were potentially accessed by hackers between mid-May and July, credit reporting agency Equifax said.
Its shares plunged after the disclosure.
The company said criminals had accessed details including names and social security numbers. Credit card numbers for about 209,000 people, and certain documents for another 182,000 were also accessed. Equifax has set up a website to help people figure out if they are among those whose information was compromised.