Abstract: This paper argues that firms may not issue debt in order to avoid the adverse selection cost of debt. Theory suggests that since debt is a concave claim, it may be mispriced when outside investors are uninformed about firms’ risk. The empirical literature has however paid little attention the caveat that the “lemons” problem of external financing first identified by Myers (1984) only leads to debt issuance, i.e. a pecking order, if debt is risk free or, if it is risky, that it is not mispriced. This paper examines whether and for what firms the adverse selection cost of debt is more than a theoretical possibility and how this cost relates to other costs of debt such as bankruptcy. Absent any direct measure of something that is unknown to investors and thus cannot be in the econometrician’s information set, we present extensive strong and robust evidence in a large unbalanced panel of publicly traded US firms from 1971 to 2001 that firms avoid issuing debt when the outside market is likely to know little about their risk.