Recent studies have analyzed social and dimensional comparisons simultaneously in order to consider their impact on students' academic self-concept (e.g., Chiu, 2012). Thereby, social comparisons refer to comparisons with the achievement level of students' classmates, whereas dimensional comparisons comprise comparisons between students' individual achievements across different domains. This paper analyzes whether both achievement comparisons influence students' subject-interest in mathematics and English (as a first foreign language). The analyses are based on N = 1390 German fifth and sixth grade students who participated in the BiKS-8-14 longitudinal study. Using multi-level analyses, results indicate that students' competences influence their mathematical and English subject-interests, demonstrating the typical pattern of social and dimensional comparisons. Further, analyses reveal mediation effects by subject-specific grades and self-concepts. These findings also apply for the development of students' subject-interest from grade 5 to grade 6. Results are discussed with respect to their implications concerning theories of achievement comparisons and interest development.