In this study, we investigated to what extent arithmetic ability and self-regulated learning skills in the beginning of lower secondary school predicts measures of students' performance in mathematics at the end of lower secondary school. Arithmetic ability and self-regulated learning skills were tested the first two weeks in lower secondary school. Post-tests were performed the last two months in lower secondary school. All testing was performed by the class teacher. A total of 219 students attending 10 different classes in ten different schools were included in the study. The 108 female students and 111 male students were all between 12-13 years old when they started lower secondary school, and 15-16 years old when they left lower secondary school. The present study shows that arithmetical knowledge is an important predictor to all competencies tested in the national test at the end of lower secondary school. The regression equation demonstrates that operation with numbers is a stronger predictor than concept of numbers to all competencies measured in the national test except complex problem solving. This study also highlights the importance of taking affective factors into account in discussions about the results of mathematics teaching and learning. It is widely reported that students self-regulated learning skills have a strong relation to achievement in mathematics which is also proven in this study. The strong correlation between operation with numbers and achievement, affective factors and achievement in mathematics helps us to identify some weaknesses in the Swedish education system. (Contains 5 tables.)