IPO volume fluctuates substantially over time. This paper compares the extent to which the aggregate capital demands of private firms, the adverse-selection costs of issuing equity, and the level of investor optimism can explain these fluctuations. Empirical tests include both aggregate and industry-level time-series regressions using proxies for the above factors and an analysis of the relation between post-IPO stock returns and IPO volume. Results indicate that firms’ demands for capital and investor sentiment are important determinants of IPO volume, in both statistical and economic terms. Adverse-selection costs are also statistically significant, but their economic effect appears small.