Last year she was named America’s richest self-made woman, with her wealth estimated at a whopping $4.5bn (£3.1bn).
But Forbes had based their assessment on Elizabeth Holmes’s 50% share of the blood-testing startup Theranos, which was valued at $9bn in 2014.
In April the company, started by the 32-year-old in 2003, confirmed that it is being investigated by several regulators following a series of Wall Street Journal reports.
The reports quoted former Theranos employees as saying that the company’s tests, which use only a few drops of blood, are unreliable.
The company said the reports were inaccurate but, nevertheless, Forbes took another look and spoke to industry experts.
Annual revenues for Theranos were revealed to be less than $100m, leading them to conclude that the company’s worth is closer to $800m, rather than $9bn.
This includes credit for the $724m it has raised, Forbes said.
Investors in the company have preferred shares, which means that, in the event of any liquidation, they get paid back before owners of common shares. Ms Holmes has common shares.
With all of that taken into account, Ms Holmes’s stake is “essentially worth nothing”, the magazine concluded.
Theranos has been contacted for comment.