September has reminded investors that the S&P 500 is heavily weighted at the top. The benchmark index declined only 0.5%, but 78% of its component stocks were down for the month.

This has become a key topic of discussion on Wall Street as many worry that the strength in the S&P 500 — which is only about 2% below its record high — is unsustainable given that the market for stocks is actually declining.

Among a sea of decliners, there were a number of standouts that lost 20% or more for the month, from various sectors including technology, financials and consumer discretionary. There were also some big winners — but the bulk of those were tech stocks.

Before listing the worst- and best-performing stocks for September, here is a look at how the 11 sectors of the S&P 500 performed, along with weighted forward price-to-earnings ratios:

The sector list is sorted by September price changes in ascending order, with the full index at the bottom. (All price changes in this article exclude dividends.) Nine of the sectors were down for the month.

Looking at the year-to-date figures, the S&P 500 has gained 11.8%, which marks another good year for overall performance. And all but three of the sectors show year-to-date gains.

But this takes us back to the index’s weighting by market capitalization. Nvidia alone makes up 8.3% of the State Street SPDR S&P 500 ETF Trust, which tracks the index by holding all of its stocks. Nvidia’s stock has gained 22.5% so far this year. Among the full S&P 500, 46% of stocks are down for 2026.

The sector table includes forward price-to-earnings ratios, which are based on September’s closing prices and consensus 12-month earnings-per-share estimates, weighted by market capitalization. For the full index, the forward P/E has declined significantly, to 19.1 from 22.2 at the end of last year. Rolling 12-month earnings estimates have been rising more quickly than share prices, on a weighted basis. The same holds true for all the sectors.

So despite a weak September for the market, investors can take some comfort that this year’s rally for the cap-weighted S&P 500 has been driven by earnings growth.

Worst-performing S&P 500 stocks in September

Through the close on Wednesday, 60% of stocks in the S&P 500 showed declines of at least 5% for September, while 27% were down at least 10% for the month.

Here are the 15 stocks that fell at the most in September:

The worst-performing stock among the S&P 500 during September, by far, was Fair Isaac. The stock sank 27% on Tuesday after Federal Housing Finance Agency Director Bill Pulte said that data from VantageScore would used as part of a unified pricing grid for federal agencies purchasing mortgage loans. That, along with the news that Rocket Mortgage will be using VantageScore as its preferred method for credit scoring, could be perceived as threats to FICO and to its rival credit agencies Equifax (also on the list above) and TransUnion, whose credit scores have been relied on for decades by U.S. lenders.

But it is important that the change announced by Pulte only affects the federal pricing model. Fannie Mae and Freddie Mac, which provide underwriting guidelines to mortgage lenders and which purchase and securitize most mortgage loans in the U.S., continue to make use of the legacy credit agencies.

Best-performing stocks in September

Here are the 15 stocks in the S&P 500 that rose the most during September:

Getting back to the importance of cap weighting for the S&P 500’s performance on the index level, Meta Platforms, Advanced Micro Devices and Intel make up a combined 4.8% of the SPY portfolio.