We show that collateral constraints restrict firm entry and postentry growth, using French administrative data and cross-sectional variation in local house-price appreciation as shocks to collateral values. We control for local demand shocks by comparing treated homeowners to controls in the same region that do not experience collateral shocks: renters and homeowners with an outstanding mortgage, who (in France) cannot take out a second mortgage. In both comparisons, an increase in collateral value leads to a higher probability of becoming an entrepreneur. Conditional on entry, treated entrepreneurs use more debt, start larger firms, and remain larger in the long run.