The Chulalongkorn University Test of English Proficiency (CU-TEP) is an institutional standardized English proficiency test designed to measure students' ability to use English for academic purposes. It is required for admission to graduate programs and undergraduate English programs at Chulalongkorn University and some other educational institutions. This study examined how accurate CU-TEP was as a predictor of the academic achievement of a total of 520 first-year Chulalongkorn University students as measured by three compulsory English course results and two English writing task scores. The participants were selected from a cohort of 5,564 freshmen taking CU-TEP in 2014, using the systematic random sampling technique. To analyze the data, multiple regression analysis was applied to assess the relationship between the CU-TEP scores and students' English course results, whereas Pearson's correlation was used to explain the correlation of CU-TEP writing scores with the writing task scores. The findings exhibited significant evidence for the validity of CU-TEP as an effective indicator of students' academic success. Also, although the predictive validity of CU-TEP was found to vary marginally according to different academic disciplines as indicated by their similarly high correlation coefficients, the discipline-specific correlations revealed a clear distinction in the predictive power of individual CU-TEP sections. Finally, correlations between the CU-TEP writing scores and the two English writing task scores confirmed the CU-TEP writing section as a relatively good predictor of students' writing ability.