International English Language Testing System (IELTS) scores have been widely used as the language entry requirements for international students in many English-medium universities. However, research on the predictive validity of IELTS on subsequent academic performance has been inconclusive. Additionally, despite the proliferation of IELTS preparation courses, very few attempts have been made to explore the extent to which these courses support course takers' development of knowledge and skills needed for their subsequent university study. This paper investigated (a) the predictive validity of IELTS results on the subsequent academic performance of Vietnamese international students at UK universities, and (b) the extent to which an IELTS preparation course in Vietnam supported its course takers in their subsequent academic study in the UK. Questionnaires with 80 Vietnamese international students from 31 universities across the UK showed that there was a positive significant correlation between these students' IELTS scores and their academic results. Follow-up interviews with two undergraduates and two postgraduates who had previously studied in the same IELTS preparation course, but were studying different academic disciplines at different institutions in the UK, and analysis of the materials used in their IELTS preparation course and university courses revealed that the course positively supported these students' subsequent academic study, but certain skills and knowledge needed in their academic study were not effectively covered in the IELTS preparation course.