The aim of this paper was to examine the relationship between Trends in Mathematics and Science Study (TIMSS) mathematics achievement and the two school achievement measures of grades and national test results in Sweden. A further aim was to examine the association of TIMSS mathematics achievement with different subgroups of students. The results show that there is a strong positive relationship between TIMSS mathematics achievement and national tests and obtained grades from both grade 6 and grade 9. Students from more educated homes performed better overall on TIMSS than those from less-educated homes, and the relationships between TIMSS and the school achievement measures were stronger for students from more educated home backgrounds. The school context and the students' background had an impact on the students' TIMSS result. The results have implications for how one should view the results from TIMSS as a measure of student mathematics achievement and thus how TIMSS results can be used in a national context.