The 25 most valuable U.S. startups that failed this year

Samantha Lee/Business Insider

Millions of dollars in backing from venture capitalists doesn't guarantee the longevity of a startup.

Even well-established private companies are at constant risk of failure, as evident by some of the startups that went out of business this year. Pitchbook compiled data on the 25 most valuable startups that failed in 2018, and three of these companies have been around for more than 20 years and were still forced to shutter.

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Startups in the healthcare industry took a big hit - seven companies on the list are in the medical sector. The list is headed by blood-testing startup Theranos, whose $9 billion valuation was greater than the valuations of all the other startups on the list combined.

Here are the 25 most valuable VC-backed startups that failed in 2018: