Standardized tests are often designed to provide only a snapshot of test takers' knowledge, skills, or abilities at a single point in time. Sometimes, however, they are expected to serve more demanding functions, one of them is assessing change in knowledge, skills, or ability over time because of learning effects.The latter is the case for the newly developed "TOEFL Junior"®Standard test, which measures improvement in young learners' proficiency in English as a foreign language. In this study,we used nonexperimental-repeated measures data from approximately 4,600 students from multiple countries to examine the extent to which observed patterns in within-individual changes in test scores were consistent with changes in underlying language proficiency because of learning. Because most students were actively participating in English language learning programs, the time interval between test administrations, which varied among students, served as a proxy for the extent of English language learning opportunities. We used hierarchical linear models to model growth in test performance as a function of the time interval between test administrations and found a positive, statistically significant relationship; that is, test takers with longer intervals between retesting exhibited greater gains than did test takers who retested at shorter intervals.The estimated relationship for the total score corresponded to between 0.16 and 0.24 test standard deviations of growth per year, depending on model specification.The findings are robust to sensitivity analyses that explore potential biasing factors.Overall, the findings are consistent with the hypothesis that the "TOEFL Junior" Standard test is capable of reflecting change in English language proficiency over time.