PRIMECAP MANAGEMENT CO/CA/ reported BIOGEN IDEC COM (BIIB) in 89 quarterly 13F filings from 2023 Q3 through 2025 Q4. Peak portfolio weight reached 3.47% in 2023 Q3. The latest visible filing shows BIIB at 1.97% of the fund.
This page tracks quarterly shares, estimated market value, portfolio weight, filing dates, and disclosure-aligned stock prices directly from SEC EDGAR 13F filings. Older history is available after unlock; the visible window still shows the most recent pattern.
This page is designed to answer the real investor question behind a 13F filing: how meaningful was PRIMECAP MANAGEMENT CO/CA/'s position in BIOGEN IDEC COM, and how did that conviction change over time? You can read the holding window, peak weight, latest reported size, disclosure timing, and quarter-by-quarter changes without piecing the story together from raw SEC rows.
BIIB was reported at 1.97% of portfolio in the most recent visible filing.
Peak weight helps separate a token disclosure from a real conviction holding the fund genuinely cared about.
Longer holding windows often tell a very different story from short-lived tactical trades or one-quarter experiments.
Knowing that a fund owned a stock is useful. Knowing whether it was a 0.20% placeholder or a true high-conviction position is where the signal actually starts.
As of 2025 Q4 — sorted by position size
These are the practical questions this page is built to answer before you even open the full history table.
PRIMECAP MANAGEMENT CO/CA/ reported BIIB across 10 quarterly 13F filings, from 2023 Q3 through 2025 Q4.
The largest reported portfolio weight for BIIB was 3.47% in 2023 Q3.
The most recent filing on this page is 2025 Q4, when PRIMECAP MANAGEMENT CO/CA/ reported 14,757,997 shares, equal to 1.97% of portfolio, with an estimated market value of $2.60B.
The chart compares PRIMECAP MANAGEMENT CO/CA/'s quarterly BIIB portfolio weight with the stock's indexed share price, so you can see whether the fund was building, trimming, or holding the position as price moved.