I measure the effect of mortgage debt-to-income restrictions on house prices using a change in the eligibility requirements imposed by Fannie Mae and Freddie Mac. I show that in 1999 Fannie Mae and Freddie Mac’s debt-to-income rules diverged, leading to tighter lending standards in places where local lenders had pre-existing relationships with Freddie Mac. Locations with tighter debt-to-income requirements experience an immediate relative reduction in house prices, showing that changes in lending standards have powerful effects. The effect builds over time and leads to a smaller house price boom and bust in these locations during the 2000s.