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When Sarah Kauss came up with the idea for a high-end water bottle company, she anticipated starting a small, niche business and then going back to a more traditional job once the new venture was off the ground.

Little did she know that that company, S’well, would become the fastest-growing women-owned business in the U.S., according to an annual ranking by the Women Presidents’ Organization (WPO), a nonprofit membership group for female leaders, in partnership with American Express.

The list, which is in its ninth year, ranks companies based on both percentage and absolute growth. To qualify for the ranking, businesses must be privately-held, woman-owned/led, and have reached revenue of at least $500,000 by the first week of 2011 and $2 million in 2015. Together, the 50 companies on the list generated a combined $4.96 billion in revenues last year.

S’well, which continues to be self-funded, had revenue of nearly $50 million in 2015, up from about $2.5 million in 2013. “We grew really, really slowly in the beginning and then really quickly in the last couple of years,” says Kauss, who was featured on Fortune’s 40 Under 40 in 2014. “And it’s because of that slow start that I got to think a lot about brand positioning and establish S’well as a luxury brand.”

Not having to answer to investors or worry about employees (Kauss was S’well’s only employee for the first couple of years after founding the company in 2010) meant she could take her time establishing the brand and educating consumers about her product, a water bottle that can keep liquids cool for 24 hours and hot for 12.

S’well is joined at the upper reaches of the list by executive search firm Pinnacle Group (No. 2) and boutique fitness chain Orangetheory (No. 3).

“We’re seeing more non-traditional women-led and women-owned businesses. More technology, more manufacturing,” says Marsha Firestone, president and founder of WPO. Yet the major characteristics of female leaders, who made up 36.2% of businesses owners in 2012, haven’t changed, she says.

The number one thing they all have in common? “They’re totally driven.”

Here is the full list of WPO’s 50 fastest-growing women-owned businesses: