The content included the phone numbers of the hotel heiress' friends.
A representative for T-Mobile confirmed Monday that information from Hilton's T-Mobile Sidekick has been posted online.
"T-Mobile's computer forensics and security team is actively investigating to determine how Ms. Hilton's information was obtained," the company said in a statement. "This includes the possibility that someone had access to one of Ms. Hilton's devices and/or knew her account password."
Theallows owners to make phone calls, surf the Web, take pictures, and send e-mail and instant messages. It uses an online server to store at least some information, including phone numbers.
The Drudge Report said it had confirmed the authenticity of many of the numbers and e-mail addresses, including those of rapper Eminem, actor Vin Diesel, actress Lindsay Lohan, singers Christina Aguilera and Ashlee Simpson, and tennis players Andy Roddick and Anna Kournikova.
The information was allegedly posted on Illmob.org on Sunday, but is no longer accessible. The FBI reportedly has opened an investigation.
The company declined to say when the breach had occurred or when it was discovered, citing an ongoing investigation.
The revelation comes a month after T-Mobile admitted that ato the names and Social Security numbers of 400 T-Mobile customers. The incident, which was discovered in late 2003, came to light after 21-year-old Nicolas Jacobsen was charged with the crime.
Jacobsen pleaded guilty Tuesday to one felony charge of accessing a protected computer and causing reckless damage. He is scheduled to be sentenced in May and faces a maximum sentence of five years' imprisonment and a $250,000 fine.