The use of students' second language as the medium of instruction in schools is quite common in former colonies of Western countries. Schools in Hong Kong have had this kind of experience for more than 150 years. In this study, three factors, namely economic, political, and educational, are found to affect the choice of medium of instruction in schools. In particular, the economic factor is found to be the dominant one for most of the time, while the educational factor has the smallest influence. Due to the change of the above three factors and their interactions over time, there were more Chinese-medium secondary schools in the first 100 years (1842-1949) and then more English-medium secondary schools during the transitional period 1950-97. However, the political factor suddenly becomes dominant after the change of sovereignty--there is an abrupt drop in English-medium secondary schools after government intervention in 1998. Educators should be more vocal and involved in politics in order to push the Government to make decisions that are educationally right. [Copyright of Language and Education is the property of Routledge. Full article may be available at the publisher's website: http://dx.doi.org/10.1080/09500780408666891]