This paper uses an optimal auction approach to investigate the conditions under which uniform pricing in IPOs is optimal. We show that the optimality of a uniform price in IPOs depends crucially on whether the (optimal) allocation rule is restricted. These restrictions may stem from the retail investors' budget constraint and/or from the institutional investors' preferences. We show that the main determinant of the optimality of a uniform pricing rule is the existence and the shape of the retail investors' budget constraint. In contrast, institutional investors' preferences are shown to mainly affect the optimal allocation rule.