HBO's online channel has grown from 2 million US subscribers to 5 million in a year, according to Bloomberg.
The boost is a result of subscription sales on Amazon and streaming TV services such as DirectTV Now, a unnamed person familiar with the matter told the publication.
The launch of HBO's online channel in 2015 was deemed risky by some industry experts, who said it could jeopardize the company's relationship with traditional TV distributors. Nonetheless, it's grown rapidly from the 2 million customers it had last February.
Online subscribers make up around 10 percent of HBO's US customers. Worldwide, the company has 142 million subscribers.
HBO's revenue increased 7 percent in 2017 thanks to more subscribers and higher prices. Along with sister channel Cinemax, it gained more than 5 million subscribers on both traditional and online platforms in the US, the largest annual growth it's had.
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