Amazon is notoriously tight-lipped when it comes to sharing numbers about its businesses. For example, it took the e-commerce giant nearly 10 years to reveal the revenue for its cloud business, Amazon Web Services.

With Amazon’s fourth quarter earnings on Thursday afternoon, many investors are keeping their fingers crossed that the company will reconsider that approach for its subscription business, Amazon Prime. Although Amazon hasn’t revealed sales or growth numbers for Prime, estimates by market researchers show that it could be the company’s next high growth business.

For $99 per year, Amazon Prime members can access streamed movies, TV shows, and Amazon’s original productions in addition to free two-day shipping and one-hour delivery on certain orders. As Scot Wingo, the executive chairman of e-commerce software company ChannelAdvisor wrote, “the Prime ‘fly wheel’ is definitely working so we’ll be listening for any tidbits on Prime and where Amazon is taking this popular program.”

According to Consumer Intelligence Research Partners report released Monday, Prime has 54 million U.S. members, up 35% from 2014. The report also showed that 47% of Amazon’s total customers are Prime members. These latest numbers follow the recent news that 38% of American households are members, according to investment bank Cowen.

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Mark Mahaney, an analyst at RBC Capital Markets, said that Prime’s growth is one of the “biggest takeaways” from Amazon’s holiday push. Amazon said in the third week of December that it had added 3 million new members without stating publicly the number of total Prime members.

Crucial to Prime’s growth is Amazon’s ability to make as many items as possible eligible for Prime’s fast shipping. With that in mind, Amazon has made a big effort to have third-party merchants use its own fulfillment centers, giving it more control so that it can ensure quick delivery.

According to Colin Sebastian of Baird Equity Research, the total number of Prime-eligible products, including those that are soled by both Amazon and third-party merchants, grew 21.2% from 2014 to 2015. There’s also opportunity for more growth in offering Prime eligible products. Sebastian writes, “Only 13% of all physical items are currently eligible for Prime shipping, suggesting a meaningful opportunity for further Prime expansion.”

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Thursday’s earnings should also bring confirmation of whether Amazon amzn had a successful holiday shopping season. ComScore estimated that total holiday e-commerce was only up 6% during the holiday season.

But Mahaney says revenue will still come in strong. He is expecting $36 billion in revenue in the fourth quarter, which would be a significant jump compared to $29.33 billion in sales the same quarter in 2014.

Stay tuned.