This paper investigated the relationship between language and mathematics achievement. Data from TIMSS-R of 6601 Filipino 7th graders were analyzed. Mathematics achievement of students who took the English version and the Filipino version of the test were compared. Results showed that students who took the English version test performed significantly better than those who took the Filipino version test in five content areas. There was a significant but weak relationship between students’ self-concept of language and mathematics proficiency and their achievement. Results suggested an investigation of the teaching and assessment practices in a bilingual classroom to identify language-related practices that contributed to students’ mathematics achievement. Cited in Online Forum (EduPhil): http://eduphil.org/forum/medium-of-instruction-and-timss-t-519.html Cited in Newspaper Column: http://opinion.inquirer.net/inquireropinion/columns/view/20090613-210240/Intellectualizing-a-language