Web site neowin.net reported that thousands of Hotmail accounts were posted online a few days ago.
And, although they've now been removed, Neowin said it has seen part of the list and has confirmed that the accounts - mostly European - are genuine. The list comprises over 10,000 accounts, said Neowin - and they only count the letters A to B.
The BBC heard back from Microsoft and said it too has seen a similar list. Microsoft said it was investigating what had happened.
Addresses include those belonging to hotmail.com, msn.com and live.com - passwords and live IDs were available at pastebin.com.
The Neowin story is here.