Officials of the University of Southern California said they will contact everyone who used the school's online application system in the past eight years to warn them that a hacker may have been able to read their files.
School security officials said they plan to contact about 270,000 people although they believe the hacker looked at only about 10 files.
"Although we believe that the scope of this is pretty small, we're taking it very seriously and we are taking great care to notify every single person where there is even the potential that their records might have been viewed," said L. Katharine Harrington, USC's dean of admission and financial aid.
The hacker took advantage of a security flaw he discovered while trying to use the USC Web site on June 20, said Robert M. Wood, USC's information security officer.
However, the hacker then reported the flaw to an online security magazine, SecurityFocus, and the publication informed USC.
Wood said the FBI was notified but he doubted that any criminal case will be pursued because there didn't appear to have been any malicious attempt to obtain private information.
FBI officials would not comment.
Since the middle of last year, computer security lapses have been reported at several other schools.
Harvard University, the Massachusetts Institute of Technology and Stanford University all rejected dozens of business school applicants who tried to access admissions Web sites earlier this year in hopes of learning their fate ahead of schedule.
A former University of Texas student was indicted last fall on charges he hacked into the school's computer system and stole Social Security numbers and other personal information from more than 37,000 students and employees. California State University, Chico, had a similar incident in March.