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The S&P 500 (^GSPC -0.52%) has climbed around 12% in 2026 as of this writing, not counting dividends. But three of the index's five biggest companies have trailed. Microsoft (MSFT +0.66%) is up around 8% this year, Amazon (AMZN -0.95%) is up about 7%, and Alphabet (GOOGL -1.24%)(GOOG -1.08%) is up about 2%, based on its Class A shares.

These three make up about 15% of the index. And they all report quarterly results within two days of each other. Microsoft and Alphabet are scheduled for Wednesday, Oct. 28, and Amazon follows on Thursday, Oct. 29.

For anyone with an S&P 500 fund, those reports are arguably the biggest scheduled test left in the fund's year. Yet this year's 12% rise has needed surprisingly little from these three stocks.

Image source: Getty Images.

The index hasn't needed them

At the end of 2025, Microsoft, Amazon, and Alphabet's two share classes made up around 15.6% of Vanguard's S&P 500 index fund, according to the fund's annual report. This was slightly more than Nvidia (NVDA -0.59%) and Apple (AAPL -0.14%), which had around 14.6% together.

The two groups have done very different things for the index, though. By my math (weighting each stock's gain this year by its share of the fund at the end of 2025), Apple and Nvidia added around 3 percentage points to the index's 12% rise, with Apple up about 28% and Nvidia about 14% as of this writing. The three laggards added about 1 point in all. Even Micron Technology and Advanced Micro Devices, which together started the year at just above 1% of the fund, contributed nearly 2 points, as Micron's shares tripled and AMD's more than doubled.

In other words, if the three laggards had simply matched the index this year, an S&P 500 fund would've gained almost 13% instead of 12%.

The fund's weights have already changed, of course. Nvidia and Apple now make up about 15.5% of the index, but Microsoft, Alphabet, and Amazon have dipped to about 15%.

How much could the reports move an S&P 500 fund?

Each of the three stocks has moved around 10% or more the day after at least one of its reports in 2026.

Microsoft dropped about 10% the day after its fiscal second-quarter report in January, then jumped about 16% after its fiscal fourth-quarter report in July. Alphabet's Class A shares climbed around 10% after the company's first-quarter results came out in April, and they fell about 7% following its second-quarter results in July. And Amazon slid about 6% after its fourth-quarter report in February before soaring about 15% when it reported in July.

For someone with $10,000 in an S&P 500 fund, around $1,500 is in those three stocks today.

If all three moved as they did after their July reports, that'd add about $110 to the fund, or a bit over 1%, assuming the rest of the index stayed flat. A repeat of how they reacted to their January and February reports would take away about $80.

Even for three stocks over two days, a move like that is big. But that's about what the whole index can move in one session. After all, the S&P 500 dropped about 1.2% on Oct. 22 alone.

The largest stocks haven't moved together

Nvidia, Apple, Microsoft, Alphabet, and Amazon make up about 30% of the index now. That much weight at the top can make an S&P 500 fund look like one big bet on a few tech giants.

This year, at least, those five have split. Two of them did much more for the index than their size alone would suggest, but the other three trailed it. The reports could shrink that gap or widen it, and I don't know which. Alphabet's Class A shares, for example, trade about 21% under their 52-week high as of this writing, while Microsoft is about 5% below its own.

A fund owner could try to guess the direction. But even a sharp reaction from all three would probably move the fund by about a point, and the fund holds them at full size whether the reports help or hurt.

I wouldn't change anything about an S&P 500 fund before Oct. 28. The three laggards are about 15% of what it owns, and the rest of the fund has done fine without them this year.