We propose material weaknesses in internal controls (MW) as a key explanation for the long-run stock underperformance following initial public offerings (IPOs). Because effective internal controls safeguard the integrity of financial data and ensure high-quality reporting, disclosure of MWs provides a negative signal about the quality and precision of accounting information. Therefore, disclosures of MW are likely to exacerbate IPO stock underperformance. Consistent with our hypothesis, we find that the magnitude of IPO underperformance is economically large and statistically significant for IPOs with MWs regardless of the holding period (one to three years). Further, the magnitude of the underperformance increases with the frequency or persistence in MWs. In sharp contrast, IPOs with effective internal controls exhibit economically small or no underperformance depending on the holding period. The results are robust to various risk controls when measuring underperformance. Overall, our findings highlight the importance of internal control quality as a key determinant of the post-IPO long-run stock performance and suggest that disclosures about the quality of internal controls provide value-relevant signals about information risk for newly public firms.