The data includes names, email addresses, phone numbers and some credit card details.
The database was published on BitTorrent last night and appears to be genuine, with some reports saying that UK government people were among those outed.
The hackers, calling themselves Impact Team, threatened to publish the data if Ashley Madison wasn’t taken down within 30 days.
The company which owns Ashley Madison said in a statement that the move was a criminal act not only against it, but against the members of its web site.
Researchers have established that the hacked data appears to be genuine.
Ashley Madison has as its motto, “Life is short. Have an affair”.