In this paper, a nationally representative longitudinal data of mathematics learning outcomes in Finland is analyzed in order to determine the direction of causality between mathematics-related affect and achievement. First, the results indicated that students' mathematical achievement, emotion, and self-efficacy were significantly stable over time. Different models were estimated to test the reciprocal relationship between students affect and achievement. The results indicated that mathematics achievement and self-efficacy have a reciprocal relation, where the dominant effect is from achievement to self-efficacy. The results indicate also a weaker unidirectional effect from achievement to emotion. [For the complete proceedings, see ED597799.]