The 2010/2011 academic year marked an important turning point in the development of Hong Kong's medium-of-instruction (MOI) policy as it offered secondary schools greater autonomy in determining their MOI policy. This paper examines the implementation of the new fine-tuning MOI policy in a representative secondary school. It compares its school-based language policy with students'(Years 7, 8 and 10) self-reported data about their actual use of English over a five-day week. At the junior secondary level (Years 7 and 8), the findings indicate a close alignment of policy and practice only in the English-medium subjects, whereas in some other subjects, the proportion of use of English could not be clearly determined due largely to the complexity of the school-based policy and teachers' flexibility and autonomy in practice. Furthermore, it is revealed that a highly sophisticated language-using situation at the senior secondary level (Year 10) poses potential challenges for the transition of students graduating from the junior level. The paper concludes by suggesting that a likely outcome of this newly implemented policy will be a return to the colonial government's laissez-faire policy in the 1980s and 1990s, where there was virtually no monitoring of policy implementation.[Copyright of Journal of Multilingual and Multicultural Development is the property of Routledge. Full article may be available at the publisher's website: http://dx.doi.org/10.1080/01434632.2013.770518]