Upcoming S&P MidCap 400 additions
Every change is announced days before it takes effect, and funds tracking the indexes must own the stock by the effective close. This page tracks that window: who is coming in, when, and how the stock is trading while the clock runs down.
Countdown to the effective close
No pending additions as of Sep 16, 2026
The index committee hasn't announced a change that is still waiting to take effect. New announcements show up here automatically with a live countdown — the most recent completed additions are below in the meantime.
Joined in the last 90 days
See what inclusion is worth →What usually happens after the bell
Measured from the effective-date close across 78 tracked S&P MidCap 400 additions.
| Window | Average | Median | Positive | vs S&P 400 |
|---|---|---|---|---|
| 1W | -1.5% | -0.6% | 45% | -1.7% |
| 1M | +2.8% | +2.3% | 56% | +1.9% |
| 3M | +8.3% | +0.1% | 52% | +4.9% |
| 6M | +4.8% | +1.0% | 51% | -1.2% |
| 1Y | +9.1% | -3.5% | 47% | -3.6% |
How S&P MidCap 400 additions work
Index committees review the S&P MidCap 400 on a published schedule and also make ad hoc changes when a constituent is acquired, delisted or no longer eligible. A change is announced first and only takes effect after the close on the effective date. Between those two dates, every fund tracking the index (MDY, IJH) has to buy the incoming stock, which is why the window matters to traders.
Across the 78 S&P MidCap 400 additions tracked here, the median notice between the announcement and the effective close is 0 days.
What happens to a stock after it joins S&P MidCap 400
Inclusion is often described as a guaranteed pop. The tracked history is more mixed: the average stock ran up +4.3% in the 30 days into the effective close, then returned -1.5% in the first week (45% of additions were positive), +2.8% after one month, and +9.1% after a year. Returns are measured from the effective close and can also be viewed against the index itself, so a rising market doesn't get counted as an inclusion effect.
Best one-year outcome in this set: ATI (ATI Inc.) at +176.0%. Worst: AVAV (AeroVironment) at -48.1%.
When are S&P MidCap 400 changes announced?
Review cadences differ by index provider. Scheduled review, announcement and effective dates for the next twelve months are listed on the index review calendar. Ad hoc changes — a takeover, a bankruptcy, a fast-track promotion — are announced outside that schedule, usually with a few days' notice.
Most recent S&P 400 additions
- SUI — Sun Communities joined the S&P 400 on Aug 20, 2026, replacing Webster Bank.
- KRYS — Krystal Biotech joined the S&P 400 on Jul 24, 2026, replacing Taylor Morrison.
- MOH — Molina Healthcare joined the S&P 400 on Jul 22, 2026, replacing National Storage Affiliates Trust.
- BTSG — BrightSpring Health Services joined the S&P 400 on Jul 17, 2026, replacing Chart Industries.
- ALSN — Allison Transmission joined the S&P 400 on Jul 6, 2026, replacing Goodyear Tire & Rubber.