Did securitization exacerbate the foreclosure crisis? I exploit the third quarter 2007 freeze of private mortgage securitization as an exogenous change in the probability of securitization for jumbo mortgages. The unanticipated freeze left banks holding mortgages that were intended for securitization at the time they were originated. Loans originated shortly before the freeze are similar to loans originated earlier in the year but were significantly less likely to be securitized. Using origination month as an instrument for securitization, I find that private securitization substantially increases foreclosure probability and decreases modification probability for delinquent loans. My estimates imply that over 500,000 of the 4.4 million foreclosures experienced since the start of the financial crisis were caused by securitization.