Mortgage dollar roll, the most common financing strategy for agency MBS, differs from repo in that the returned collateral can differ from those received. Also, MBS ownership changes hands in the funding period. We show that dollar roll “specialness,” how much implied financing rates fall below MBS repo rates, (1) increases in the value of the cheapest-to-deliver option, (2) decreases in the leverage of primary dealers, (3) decreases in prepayment risk exposure during the financing period, and (4) decreases in MBS returns. The Federal Reserve’s dollar roll sales in quantitative easing operations are associated with lower specialness. Received February 3, 2016; editorial decision July 30, 2018 by Editor Itay Goldstein.