HBO has had some of its programming stolen - including full episodes of new shows that are yet to screen, and an upcoming Game of Thrones script.
In what is being described as a cyber incident, the company says it is working with law enforcement and cybersecurity firms.
HBO chief Richard Pepler told the network's employees that "proprietary information" was stolen in the hack.
"HBO recently experienced a cyber incident, which resulted in the compromise of proprietary information," the company said.
"We immediately began investigating and are working with law enforcement and outside cybersecurity firms. Data protection is a top priority at HBO, and we take seriously our responsibility to protect the data we hold."
Spokesman Jeff Cusson would not reveal which specific TV episodes, movies or other video the hackers made off with.
But Entertainment Weekly reported unreleased episodes of Ballers and Room 104 had already leaked, as well as parts of a script that resembled an upcoming episode of Game of Thrones. Up to 1.5 terabytes of data was reportedly stolen.
Gizmodo reported this email was sent to reporters on Sunday night: "Hi to all mankind. The greatest leak of cyber space era is happening. What's its name? Oh I forget to tell. Its HBO and Game of Thrones......!!!!!! You are lucky to be the first pioneers to witness and download the leak. Enjoy it & spread the words. Whoever spreads well, we will have an interview with him. HBO is falling."
Hacking Hollywood can have significant repercussions. Sony struggled in the aftermath of its huge hack in 2014, which leaked employee emails as well as films.
In April, a hacker claimed to have released episodes of Netflix's Orange is the New Black before their release by the streaming site.
- With AP