Journal of Linguistics and Language Teaching (JLLT) Though millions of students study in intensive language programmes, expected gains and the effects of widely accessible factors such as linguistic distance, L2 proficiency, sex, age, and learning contexts remain unclear. Using the ACTFL proficiency scale, this study analysed the speaking and writing development of 2,325 Novice Mid to Advanced Mid English language learners (CEFR pre-A1 to C1) over a 15-week semester in an intensive English programme. Foreign Service Institute language categories and the Automated Similarity Judgment Program were used to determine linguistic distance. The results show that on average, students progressed approximately one-third of a sublevel for speaking and writing and that only lower-proficiency learners (Novice Mid to Intermediate Low) advanced a full proficiency sublevel. Though linguistic distance and age had a slight impact on language development, initial proficiency exerted a substantially stronger effect, with large gains for lower-proficiency learners compared to higher-proficiency students, who slowed or regressed in their measured proficiency. Implications for interpreting and addressing advanced learner needs are discussed.