In Iran, many changes have been made in English language teaching. In academic grounds it is important to know the correlation between the expertise of students in English and their scores in different scientific subjects, before applying any further curriculum reform. The present study was designed to determine the correlation between English language proficiency and the achievement of medical students in their national comprehensive basic science exam (NCBSE). One hundred and fifty six students admitted in the academic years of 2008 and 2009 (60% female and 40% male) were enrolled for the present study. The results of General English and Scientific language courses, the average marks in five consecutive semesters and the NCBSE scores were extracted from their educational files. Data was analyzed using appropriated tests such as ANOVA, Pearson's correlation and liner regression. The present data indicated that proficiency in English could significantly influence academic achievement of medical students. Furthermore, changes of policy in the students' selection or teaching styles without preparation of necessary backgrounds including increasing their English knowledge could not significantly alter the achievement of Iranian medical students.