Amazon.com AMZN is poised to become the top retailer in yet another category: clothing.

The online retailer, a relative latecomer to the world of fashion and apparel, will sell $27.77 billion worth of apparel in the U.S. in 2017, enough to supplant Macy’s M as the No. 1 seller of clothes in the United States, according to a report by Wall Street firm Cowen & Co released on Monday. Just four years ago, Macy’s apparel sales were five times greater than Amazon’s. This year, Cowen expects Amazon’s apparel business to reach $16.34 billion, compared to $22.2 billion for Macy’s.

The secret behind Amazon’s ascent is its now much bigger variety of apparel on offer, something that also threatens retailers like Walmart WMT and Target TGT . Cowen estimates that Amazon sells 343,000 different kinds of items (this number includes different size and color iterations of a given item) in leading brands, compared to 292,000 for Walmart, 85,000 apiece for Macy’s and Nordstrom JWN , and 35,000 at Target. Cowen analysts credit Amazon’s big efforts in the last few years to court the big name apparel makers.

And all of that spells new trouble from Amazon for middle of the road department stores and general merchandise stores, many of whom have already paid a big price for their slow reaction to the rise of e-commerce last decade. And data suggests those retailers’ customers are starting to embrace Amazon as a credible place to buy duds: in the first half of 2015, Cowen found that 11% of Target and Walmart clothing shoppers also shopped for apparel at Amazon, up three percentage points from a year earlier.

“We see non-differentiated retailers at greatest risk, with longer-term risks to department stores & broadlines. Key risks to traditional retail: greater price transparency driving merchandise margin pressure, conflicting vendor/ supplier relationships,” Cowen analyst Oliver Chen wrote in the note, adding that Amazon’s distribution infrastructure and speedy delivery compared to many brick-and-mortar competitors and the loyalty that Amazon’s Prime subscription service is fostering are only making things harder.

Cowen adds that long-term winners will be brands with the most innovative product/category leadership, retailers that integrates stores and their web-site and those that already have a big e-commerce business.

It’s easy to see why Macy’s, Walmart, Target, Nordstrom, et al are falling over themselves to build up their e-commerce; Cowen expects Amazon’s apparel sales to have risen 26% a year between 2011 and 2020, compared to a paltry 2.5% pace for the industry as a whole.