Conventional wisdom holds that an incumbent bank would never voluntarily share proprietary borrower data with a competing lender. We study lending competition in which the incumbent owns data that generate informative signals about borrower quality and chooses how much to make portable to an entrant. When the data identify high-quality borrowers, sharing softens the entrant’s bidding after unfavorable signals more than it sharpens competition after favorable ones, so the incumbent shares voluntarily, especially for observably creditworthy borrowers. A utilitarian planner instead favors greater sharing for weak borrowers. The economic content of the data, and who controls it, jointly shape the welfare effects of open banking.