This study aims to explain the relationship between student performance in the International General Certificate Secondary Education (IGCSE) and the International Baccalaureate Diploma Program (IBDP) as well as explore the predictors of the IBDP scores in languages, sciences, and mathematics. A purposive sample of 338 high school students was drawn from a private school implementing international programs in Turkey. The data consisted of external examination scores of students enrolled between the years 2010 and 2019. Data were analyzed for correlation and with a multiple regression approach. Strong, moderate, or weak relationships were found between the IBDP exam scores and the IGCSE exam scores with regard to the different subjects. From 4% to 65% of the variation in different IBDP subject scores can be explained by the simple linear regression model in different IGCSE subject scores. These findings will help international educators to make informed decisions about teaching and learning pedagogy in the international programs.