Jan. 27 produced something one rarely sees in the stock market.
One stock fell so hard that its loss caused the Dow Jones Industrial Average to fall about 409 points, or 0.8%, to 49,003.
The culprit was health-insurance giant UnitedHealth Group (UNH), which fell 68.94 points, or 19.6%, to $282.70. A primary reason was a Trump administration proposal to keep Medicare rates roughly flat next year, something the industry and investors didn’t expect.
In addition, the company forecast a decline in revenue in 2026. That would be the first annual revenue slip in more than three decades, Bloomberg noted. The insurer has been struggling to rebuild confidence with investors.
UnitedHealth caused so much havoc in the Dow Jones that the mechanics behind it warrant an explanation.
The Dow is a price-weighted, narrow index with 30 components. So, the higher a stock’s price, the greater its impact on the average.
To figure out how a stock affects the average, you divide its daily change by a divisor. The divisor on Wednesday was 0.16242563904928.
So, UnitedHealth’s price change translated into a 424.44-point decline in the Dow. Which means the remaining 29 stocks were up a not-so-whopping 15 points.
Well, not exactly. The top six Dow point gainers on Jan. 27 — Goldman Sachs, Caterpillar, Microsoft, Home Depot, American Express and Amgen — contributed 168.8 points to the average.
The remaining Dow stocks, including Apple, Nvidia, Amazon.com and Chevron, fell a combined 153.3 points.
UnitedHealth could rise slightly on Jan. 28. So, too, could Humana.
The Nasdaq-100 Index and S&P 500 may open slightly higher, according to overnight futures trading. The Dow looks headed to a flat day.
Gold and silver prices were higher in overnight markets after a late-Jan. 27 pullback that may have been profit-taking. Crude oil was higher on Iran worries.
Jan. 28 is an exceedingly busy day for financial news. Events include:
The Federal Reserve‘s decision on interest rates. The decision is due at 2 p.m. ET, followed by a news conference with Chairman Jerome Powell. (Most Fed watchers see the Fed keeping its key interest rate at 3.5% to 3.75%. )
Earnings from key companies, including Microsoft, Meta, Tesla and IBM.
With Wednesday’s tumble, UnitedHealth has caused the blowout phenomenon twice.
On Feb. 21, 2025, UnitedHealth shares fell because of reports that the Justice Department was investigating the company for antitrust violations. The damage: a loss of $64, or about 421 Dow points, at the day’s open. It finished the day down $36, scraping more than 237 points from the average.
The investigation so far has led the company to abandon a major acquisition, and the probe is continuing.
The Dow overall fell 478.6 points that day due to additional concerns: economic slowing and the damage that President Trump’s tariff threats might cause.
The Dow is vulnerable if one of its high-price stocks crashes.
The most expensive stock in the average on Jan. 27 was Goldman Sachs, at $927.72, followed by Caterpillar at $638.91 and Microsoft at $480.48.
Verizon and Nike are the lowest-priced Dow stocks at $39.32 and $63.09, respectively.
UnitedHealth Getty Images ·Getty Images
Amazingly, UnitedHealth was not the worst-performing stock in the Standard & Poor’s 500 Index on Jan. 27. That distinction went to competitor Humana (HUM), which fell $55.70, or 21.1%, to $207.93. But the culprit was the same: the Administration proposal on Medicare rates.
And the S&P 500’s five-worst performers on the day were all managed-care or health-insurance companies.
The S&P 500, however, has 500 stocks, and it finished the day up a modest 28.4 points, or 0.9%, at a record 23,817.10. It hit an intraday record of 28,865.
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The fuel for the overall rally was gains in our old friend, technology stocks, led by Corning, up 15.6% on news that it will sell about $6 billion in fiber-optic cables to Facebook-parent Meta Platforms. Meta needs the cable as it expands its network at artificial intelligence data centers.
The S&P 500 technology sector rose 1.42% on the day, the top performer among the index’s 11 sectors. Health Care, needless to say, was the laggard, down 1.66%. Thirty-four of 60 stocks in the sector were lower.
But it was just one of two sectors declining on the day. The other was financial stocks.
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It’s important for all investors to remember how the Dow Jones Industrial Average differs from the Standard & Poor’s 500 Index (and its related siblings) and the Nasdaq Composite Index.
The S&P 500 and Nasdaq are market-cap-weighted. So, the bigger the market cap, the more influence a stock has on the indexes.
Hence, any change in Nvidia (with a market cap of $4.5 trillion) will have a much bigger impact on both indexes than, say, a change in Costco Wholesale (market cap $430.8 billion).
In fact, according to S&P data, Nvidia represented 7.76% of the entire market cap of the index on Dec. 31, 2025. The market caps of the top 10 S&P 500 stocks totaled $22.9 trillion on that date, representing 39.2% of the total market cap. And that was after a slight decline in December.
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