We study how prime brokers' credit supply affects hedge fund leverage, portfolio allocation, and performance. We construct a novel, fund-level measure of credit supply by linking survey responses on leverage availability with reported creditor relationships. Hedge funds with better access to credit subsequently borrow more and generate higher risk-adjusted returns. This effect is stronger for funds with concentrated prime broker relationships and those relying on secured borrowing rather than derivatives for leverage. Credit supply matters most during periods of market stress and when arbitrage opportunities are abundant. We also find that prime brokers allocate scarce credit disproportionately to their most profitable clients. Our findings provide empirical support for models of leverage constraints in which less constrained investors hold higher-alpha portfolios, and help explain the outperformance of large hedge funds with diverse credit sources.