We explain the clustering of underpricing in initial public offerings (IPOs). The model features an industry with aggregate demand uncertainty and asymmetric information about firms' quality. In the IPO market, firms can signal quality by underpricing or under-issuing new shares. Expected aggregate demand for the industry's products increases with the publicity that the industry creates through IPO underpricing. We show that asymmetric information and expectations on aggregate product demand interact with each other to generate multiple equilibria. Underpriced IPOs cluster in one equilibrium but not in the other. We use these results to explain why the clustering often occurs in particular industries, is short-lived, and is sensitive to economic conditions.