This paper examines the relation between audit tenure and audit quality in the presence of information asymmetry among the stockholder, the manager, and the auditor. Using a two-period adverse selection and moral hazard model, we show that even when managers are inclined to overstate earnings, auditor tenure is positively associated with audit quality as long as no asymmetric information exists between the auditor and the stockholder. However, when an auditor has hidden information about his/her efficiency, this positive association disappears and audit quality is lower than the socially optimal level. Moreover, moral hazard will lead to a suboptimal audit quality only when the problem of adverse selection exists at the same time. Finally, our result suggests that mandatory rotation is not a cure for improving audit quality.