English, the Language of Instruction (LOI), has fallen victim of blame of some scholars as the cause of poor performance in secondary schools in Tanzania. The blame is directed at English strongly up until recent years (2010 and 2011) when the results of national examinations have been extremely worse. It seems, the public presumes that there is relationship between students' academic success and their abilities in the LOI. This paper sought to investigate whether there is relationship between English Language Proficiency (ELP) and academic performance in Tanzanian secondary schools. The data used were obtained by administering an ELP test and a review of students' reports, and were quantitatively analyzed using a computer software, Statistical Package for Software System (SPSS) version 18. The study revealed that there is a significant weak positive relationship between ELP and students' academic achievement. The relationship was significant in English and insignificant in other subjects which were investigated. On the basis of the findings of this study, it is argued that academic success is a function of several variables and not only proficiency in the LOI. It is therefore recommended that the responsible authorities should invest in improving English language proficiency among students since ELP is positively connected with academic success.

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