This study analyzes firms' earnings management and the effects of earnings management on subsequent period performance. To extract samples suspected of upward earnings management, the study centers on seasoned equity offering (SEO) events, empirically examining whether earnings management occurs prior to SEOs and the performance thereafter. In addition to accruals, real activities manipulation is added as an earnings management method for analysis. Using a sample of 381 SEOs during the research period from 1995 to 2007, the study examines whether earnings management behavior through accruals and real activities exists, and analyzes post-earnings-management operating performance using univariate and multivariate regression analyses. The results show that managers utilize real activities manipulation in addition to accruals to increase earnings in the fiscal year immediately preceding the SEO, and operating performance declined significantly after earnings management. The poor operating performance following earnings management persisted only up to the second year when analyzing the full sample of firms, due to the reversal of accruals; however, firms classified in the real activities earnings management group experienced prolonged poor operating performance following the SEO. Notably, firms with low largest shareholder ownership stakes, where internal controls are expected to be relatively weak, also exhibited earnings management for SEOs, and poor performance persisted even after earnings management. Accordingly, it was confirmed that the largest shareholder's ownership stake acts as a factor in earnings management, negatively affecting subsequent period performance. While prior research has not shown consistent results regarding the timing of earnings management before SEOs, this study was able to confirm a consistent earnings management timing by additionally examining abnormal real activities as a proxy for earnings management. Furthermore, this paper's significance lies in verifying that poor performance following earnings management for SEOs is caused not only by the previously known discretionary accruals but also by abnormal real activities. Although these empirical findings are limited in that the proxies for earnings management rely on estimation models, they are expected to contribute to research on managers' motivations for choosing earnings management methods and future research related to real activities.