Rehypothecation and Collateral Reuse
When you post securities as collateral to a prime broker, the broker is often allowed to re-pledge them to someone else — a chain of reuse that funds the financial system cheaply but turns your broker's solvency into your problem.
Rehypothecation is a prime broker's right to take collateral or securities a client posted to it — say, to secure a margin loan — and re-pledge those same assets for its own borrowing or financing needs, rather than just holding them idle in a vault. It is standard, disclosed, and contractually agreed in most prime brokerage relationships, not a hidden abuse: it's the mechanism that lets brokers offer cheap financing in the first place, since they can fund themselves against client collateral instead of tying up their own balance sheet.
The economics work because the same pool of securities can support financing at more than one link in a chain. A hedge fund posts stock to its prime broker as margin; the prime broker rehypothecates that stock to raise its own funding from a bank; the bank may in turn use it elsewhere. Every link earns a spread, and the system as a whole runs on less collateral than if each participant needed fully segregated assets — but it also means the hedge fund's original collateral is now sitting somewhere it doesn't directly control, subject to whatever happens to every intermediary in the chain.
Worked example
A hedge fund posts $100m of securities to its prime broker as margin for a leveraged long position.
| Step | What happens |
|---|---|
| Fund → Prime broker | $100m securities posted as collateral for margin financing |
| Prime broker → Bank | Broker rehypothecates the same securities to raise its own funding, at a lower rate than posting cash |
| Bank | Holds the securities as its own collateral, potentially reuses them again |
If the prime broker fails while the securities are rehypothecated, the fund is no longer simply a secured lender with a claim on its own specific assets sitting untouched in a vault — it typically becomes an unsecured creditor for the value of what was rehypothecated, standing in line with everyone else in the bankruptcy, precisely because the assets themselves are now somewhere down the chain. This was a real, painful lesson for hedge funds whose UK-regulated prime broker entities were rehypothecating collateral before the 2008 Lehman collapse.
Rehypothecation converts what feels like a fully segregated custody arrangement into an unsecured credit exposure to your prime broker, up to the contractual limit on how much of your assets it's allowed to reuse.
"My broker holds my collateral" and "my broker has re-pledged my collateral to someone else" are very different risk positions, and the difference is invisible day-to-day — it only becomes visible, often too late, if the broker itself gets into trouble.
Regulators cap how much of a client's assets a broker can rehypothecate in some jurisdictions (the UK limits it to 140% of a client's debit balance), but the US has historically allowed broader reuse, which is one reason where a fund's prime broker is domiciled matters for its practical counterparty risk.
Further reading
- IMF Working Paper, Rehypothecation (Singh & Aitken)