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.