Secondary school-leaving English examination results are often regarded as indicators of students' competence to study in English-medium universities, which is usually demonstrated through source-based academic writing. In English as a second or foreign language (ESL/EFL) contexts, many English-medium universities require local undergraduates to enrol in an academic writing course, unless they received outstanding English results in secondary school-leaving examinations. This study investigates the relationship between ESL undergraduates' secondary school-leaving English examination results and their academic writing performance through latent profile analysis. Results show that students can be grouped into four classes of academic writing performer (AWP), namely Complex-AWP, High-AWP, Medium-AWP and Low-AWP. Surprisingly, the Complex-AWP group had the highest means in structure, argument and language, but the lowest in citation. Secondary school-leaving English examination results can generally predict students' class membership in language, argumentation and, to some extent, structure, but not citation. An important implication is that students with high English proficiency do not necessarily do well in all aspects of academic writing. This study can inform university senior management on how to set policies about who needs an academic writing course and provide appropriate training in various aspects of academic writing for university students with diverse English proficiency.