Roku is currently the No. 1 connected-TV device brand in the U.S., topping Google Chromecast, Amazon Fire TV and Apple TV in terms of total users, according to a new study from research firm eMarketer.

In 2017, 38.9 million Americans will use a Roku at least once per month, up 19% over last year, eMarketer estimates. Chromecast comes in second with 36.9 million users, followed by Amazon Fire TV with 35.8 million.

Apple TV comes in a distant fourth: The tech giant’s internet set-top device will have 21.3 million users. Over the next few years, Apple TV will continue to lag further behind its three rivals, eMarketer projects.

One big roadblock for Apple TV is its premium pricing. The fourth-generation Apple TV devices currently start at $150, whereas Google, Amazon and Roku each sell streaming sticks priced below $40.

Roku, on the other hand, is the only one of the four manufacturers in the segment that does not have an affiliated content service. “Roku has used its neutrality to strike deals with a wide range of partners, including smart TV makers, [over-the-top] service providers and social media companies,” said Paul Verna, eMarketer’s principal video analyst.

Roku in 2017 will capture 23% of the total U.S. connected-TV user population, followed by 22% for Chromecast, 21.3% for Amazon Fire TV and 12.7% for Apple TV. (The research firm noted that there is overlap in market penetration, as some consumers and households use more than one streaming device.)

By 2021, Roku will have added another 30 million monthly users, to continue to lead the field with a total U.S. user base of 69 million, per eMarketer’s forecast. Google Chromecast will hit 64.6 million and Amazon Fire TV will reach 62.6 million. Over that same period, Apple TV will add less than 4 million U.S. users, to reach 25.2 million by 2021, eMarketer predicts.

It’s important to note that eMarketer is estimating the total number of users per platform, not unit sales. This month Roku said it has more than 15 million active monthly user accounts; the eMarketer estimates are higher because each account may have multiple individual users.

Roku reportedly is planning to file for an initial public offering later this year, seeking a valuation of $1 billion. Earlier this year, the company said it generated close to $400 million in revenue in 2016, with more than $100 million from its media and licensing business.

All told, 168.1 million U.S. consumers will watch streaming video via a connected TV device this year, up 10% over 2016, according to eMarketer. Overall, smart TVs will capture the most streaming-video viewers with 81.2 million U.S. users this year, followed by game consoles with 62.8 million users. Blu-ray Disc players will have 29.0 million internet-video users in 2017.

As with all eMarketer forecasts, the connected-TV study aggregates data from “the most trusted sources,” according to the company. Estimates are based on the analysis of survey and traffic data from research firms and regulatory agencies, sales projections, historical trends, company-specific data, and demographic and socioeconomic factors. Media conglomerate Axel Springer acquired eMarketer in 2016.