Washback, or the influence of testing on teaching and learning, has received considerable attention in language testing research over the past twenty years. It is widely argued that testing, particularly high-stakes testing, exerts a powerful influence, whether intended or unintended, positive or negative, on both teachers and learners. This article investigates the washback effects of a high-stakes English language proficiency test, the Test of English as a Foreign Language Internet-Based Test (TOEFL iBT), in Vietnam. Vietnam, a developing country whose educational philosophies differ from those underpinning the TOEFL iBT, provided a unique context to explore the test's washback. In the course of this study, four teachers were observed and teaching materials were collected from educational institutions in Vietnam. The study revealed that the TOEFL iBT influenced both what the teachers taught and how they taught but its effects were mediated by the use of test preparation materials.