The study analyzes the effects of grade repetition on science and mathematics achievement and on self-concept in mathematics using longitudinal data from a representative sample of 9th graders in Germany. Same-age comparisons were applied between three groups: (a) the retained students, (b) a matched group of promoted students, and (c) the entire group of promoted students. The results show that the retained students have a higher self-concept in mathematics after one year of retention than before. Summarizing these empirical findings concerning achievement, we conclude that retention in the 9th grade does not reduce the gap in academic achievement to the regular promoted students.