The break-in comes just as the swelling ranks of the unemployed are turning to sites like Monster.com to look for work.

The company disclosed on its Web site that it recently learned its database had been illegally accessed. Monster.com user IDs and passwords were stolen, along with names, e-mail addresses, birth dates, gender, ethnicity, and in some cases, users' states of residence. The information does not include Social Security numbers, which Monster.com said it doesn't collect, or resumes.

Monster.com posted the warning about the breach on Friday morning and does not plan to send e-mails to users about the issue, said Nikki Richardson, a Monster.com spokeswoman. The SANS Internet Storm Center also posted a note about the break-in on Friday.

USAJobs.com, the U.S. government Web site for federal jobs, is hosted by Monster.com and was also subject to the data theft. USAJobs.com also posted a warning about the breach.

Monster.com has been checking for misuse of the stolen information but hasn't yet found any, it said. It has made changes since discovering the break-in but won't discuss them because it doesn't discuss security procedures publicly and because it is still investigating the incident, Richardson said.

She also would not disclose the volume of data stolen, but said the company decided it would be prudent to alert all of its users via its Web site.

The company advised users to change their passwords and reminded them to ignore e-mails they may get that purport to be from the company and that ask for password information or instruct the user to download anything.

Monster.com was also hit by hackers in mid-2007. At that time the hackers obtained log-in credentials for companies seeking employees and used them to access Monster.com's database of job seekers. An automated Trojan then transmitted the personal information to a rogue server. Monster.com users were then targeted with scams via the stolen e-mail addresses.

In addition, the Monster.com site was the subject of an attack that same year that inserted malicious code onto certain pages of the site, automatically downloading a virus onto computers that visited the pages.