The United States is the largest apparel market in the world, consuming about 30% of the output of thousands of apparel manufacturers—and millions of their workers—around the globe. And more of America’s retailers are dedicated to selling apparel than any other product line.

So when there’s a sea change in the way apparel is sold, it’s a very big deal. According to Internet Retailer’s just-released research report, that transformation has begun as billions of dollars of apparel sales are moving from physical stores to the internet.

That report, titled “Behind the Online Apparel Boom,” reveals that U.S. websites last year sold an estimated $80 billion in apparel, an increase of 19.7% from the prior year, growth that’s fully five percentage points greater than the country’s total e-commerce market. By comparison, U.S. store sales of apparel grew a mere 1.1% to an estimated $375 billion in 2015.

With apparel sales surging online and sagging in stores, the web’s share of 2015 apparel sales rose to 17%, up from 14.8% in 2014. By comparison, e-commerce last year accounted for 10.6% of retail sales when factoring out items not normally bought online, such as fuel and automobiles, according to the U.S. Commerce Department. Not since Amazon.com Inc. began cannibalizing bookstores in the 1990s and Apple Inc.’s iTunes did the same to music stores a decade later, have web-only merchants gobbled up such a significant share of a major retail market at the expense of stores.

Much of the growth of the online apparel market share is attributable to the massive expansion of Amazon’s marketplace, through which Amazon markets and processes purchases of apparel for hundreds of mostly small and midsized apparel retailers and manufacturers, taking a cut of each transaction. Created in 2002, apparel and accessories SKUs and sales on Amazon’s marketplace have soared in recent years, and last year alone grew 48% to $16.3 billion, according to research from Cowen Group.

Amazon’s apparel sales growth came largely at the expense of stores, not from other apparel e-retailers. The 250 apparel and accessories merchants included in Internet Retailer’s Top 1000 database, which ranks America’s largest e-retailers based on 2015 web sales, grew their combined revenues by 14%. That roughly equals the average growth rate for all e-retailers and suggests that Amazon’s apparel sales growth was incremental to—not detrimental to—online sales of other apparel merchants.

The Internet Retailer research report analyzes the sales, marketing and shopper trends in the online apparel market through a detailed analysis of the 250 apparel e-retailers ranked in the Top 1000 database.

Included in the downloadable PDF report are:

• Rankings and 2015 web sales growth rates of the largest 250 apparel merchants in North America
• In-depth charts and tables that show the total volume of apparel sales in the United States in the last two years, as well as the e-commerce penetration
• A feature on the impact of Amazon’s growing apparel business, including an in-depth breakdown of the number of apparel SKUs and sellers on Amazon
• Data and analysis that shows the growth rates and trends of each segment of the online apparel market, including men’s fashion, shoes, outerwear and lingerie
• Analysis of the marketing, pricing and merchandising strategies of the leaders in online apparel