This study examined the short- and long-term effects of self-enhancement (i.e., overreporting of academic grades) on academic self-concept and academic achievement. A total of 916, 719, and 647 students participated in the first, second, and third waves of assessment, respectively (mean age at T1 = 15.6 years). At each assessment, students reported their last midterm grades and their self-concepts in mathematics, German, English, and French. Actual midterm grades were obtained from the school administrations. Results showed that self-enhancement was positively associated with self-concept in the short term. However, in the long term, self-enhancement was directly associated with stronger decreases in self-concept and indirectly with stronger decreases in achievement that were linked to inflated self-concepts. Implications for research and educational practice are discussed.