The debate on the differential effects of mathematics textbook series is a recurrent topic in the research literature. Research results remain inconclusive, pointing to a lack of evidence to decide on the relevance of the selection by schools of a mathematics textbook series. Studies also point to difficulties in comparing textbooks. Recently, in order to examine the influence of mathematics textbook series on student learning, the need to take into account mediating variables between the mathematics textbook series and the enacted curriculum is stressed. This paper focuses on one such mediating variable: teachers' views of mathematics textbook series. Views of a mathematics textbook series of 814 teachers and mathematics performance results of 1579 students were analysed. The results point out that, with regard to teachers' views of mathematics textbook series, the question "Does it really matter which mathematics textbook series schools choose" has to be answered positively. Implications of the findings are discussed. (Contains 7 tables.)