Having 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 evidenced 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; three of these companies have been around for more than 20 years and were still forced to shutter.

Startups in the healthcare industry took a big hit -- seven companies on the list are in the medical sector.

The list is headed by Theranos, the blood-testing company, whose $9 billion valuation was greater than those of all the other startups on the list combined.

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

25. SDCmaterials -- automobile nanotechnology

25. SDCmaterials -- automobile nanotechnologyBill Pugliano/Getty Images

Year founded: 2004

Maximum valuation: $48 million

Amount raised: $26 million

Read more about SDCmaterials on PitchBook.

24. Senzari -- music and entertainment data intelligence

24. Senzari -- music and entertainment data intelligenceSean Gallup/Getty Images

Year founded: 2010

Maximum valuation: $52 million

Amount raised: $13 million

Read more about Senzari on PitchBook.

23. Industrial Origami -- industrial material manufacturer

23. Industrial Origami -- industrial material manufacturerSean Gallup / Getty Images

Year founded: 2003

Maximum valuation: $58 million

Amount raised: $41 million

Read more about Industrial Origami on PitchBook.

22. Claritas Genomics -- pediatric genetic testing

22. Claritas Genomics -- pediatric genetic testingMichael Dalder/Reuters

Year founded: 2013

Maximum valuation: $60 million

Amount raised: $39 million

Read more about Claritas Genomics on PitchBook.

21. Apprenda -- cloud-based developer software

21. Apprenda -- cloud-based developer softwareApprenda

Year founded: 2007

Maximum valuation: $90 million

Amount raised: $56 million

Read more about Apprenda on PitchBook.

20. Innovari -- interactive energy platform for the utility industry

20. Innovari -- interactive energy platform for the utility industryReuters/Kim Kyung-Hoon

Year founded: 2011

Maximum valuation: $94 million

Amount raised: $12 million

Read more about Innovari on PitchBook.

19. DataTorrent -- streaming data platform

19. DataTorrent -- streaming data platformFlickr / Leonardo Rizzi

Year founded: 2012

Maximum valuation: $96 million

Amount raised: $24 million

Read more about DataTorrent on PitchBook.

18. Rennovia -- bio-based chemical production

Year founded: 2009

Maximum valuation: $99 million

Amount raised: $69 million

Read more about Rennovia on PitchBook.

17. Navdy -- heads-up displays for vehicles

Year founded: 2012

Maximum valuation: $100 million

Amount raised: $80 million

Read more about Navdy on PitchBook.

16. EZhome -- subscription-based lawn-care services

16. EZhome -- subscription-based lawn-care servicesREUTERS/John Sommers II

Year founded: 2014

Maximum valuation: $102 million

Amount raised: $20 million

15. Winx -- sleep-therapy systems

15. Winx -- sleep-therapy systemsYouTube/Winx Sleep Solution

Year founded: 2007

Maximum valuation: $115 million

Amount raised: $77 million

Read more about Winx on PitchBook.

14. Alphabet Energy -- thermoelectric generators

14. Alphabet Energy -- thermoelectric generatorsREUTERS/Adam Fenster

Year founded: 2009

Maximum valuation: $118 million

Amount raised: $78 million

Read more about Alphabet Energy on PitchBook.

13. Paieon -- medical imaging technology

13. Paieon -- medical imaging technologyShutterstock

Year founded: 2000

Maximum valuation: $136 million

Amount raised: $34 million

Read more about Paieon on PitchBook.

12. Candescent Health -- radiology software

Year founded: 2009

Maximum valuation: $145 million

Amount raised: $94 million

Read more about Candescent Health on PitchBook.

11. Primary Data -- automation software platform

11. Primary Data -- automation software platformPrimary Data

Year founded: 2013

Maximum valuation: $150 million

Amount raised: $89 million

Read more about Primary Data on PitchBook.

10. Fallbrook Technologies -- automatic shifting technology for bicycles

10. Fallbrook Technologies -- automatic shifting technology for bicyclesEnviolo

Year founded: 1998

Maximum valuation: $169 million

Amount raised: $171 million

Read more about Fallbrook Technologies on PitchBook.

9. ReVision Optics -- surgical implants for vision loss

Year founded: 1996

Maximum valuation: $183 million

Amount raised: $175 million

Read more about ReVision Optics on PitchBook.

8. Medical Simulation -- simulation products for medical training

8. Medical Simulation -- simulation products for medical trainingSynapse/Creative Commons

Year founded: 1998

Maximum valuation: $194 million

Amount raised: $55 million

Read more about Medical Simulation on PitchBook.

7. Airware -- drone analytics

7. Airware -- drone analyticsAirware

Year founded: 2011

Maximum valuation: $59 million

Amount raised: $104 million

Read more about Airware on PitchBook.

6. ItsOn -- cloud-based platform for smart mobile services

Year founded: 2008

Maximum valuation: $243 million

Amount raised: $41 million

Read more about ItsOn on PitchBook.

5. Shyp -- on-demand delivery platform

5. Shyp -- on-demand delivery platformShyp

Year founded: 2013

Maximum valuation: $275 million

Amount raised: $62 million

Read more about Shyp on PitchBook.

4. Rethink Robotics -- robots for the manufacturing industry

Year founded: 2008

Maximum valuation: $291 million

Amount raised: $150 million

Read more about Rethink Robotics on PitchBook.

3. Videology -- TV and video advertising software

3. Videology -- TV and video advertising softwareShutterstock

Year founded: 2007

Maximum valuation: $311 million

Amount raised: $233 million

Read more about Videology on PitchBook.

2. Lytro -- full-spectrum light-field cameras

2. Lytro -- full-spectrum light-field camerasLytro

Year founded: 2006

Maximum valuation: $360 million

Amount raised: $202 million

Read more about Lytro on PitchBook.

1. Theranos -- blood-testing technology

Year founded: 2003

Maximum valuation: $9 billion

Amount raised: $910 million

Read more about Theranos on PitchBook.

--This post originally appeared on  Business Insider.