Abstract This study compares how lecturers signal important points with metadiscursive importance markers (e.g., the point is; remember) when they lecture in English as their first language (L1) as opposed to English as an additional language (English-medium instruction, EMI). Importance marking is a feature of discourse structuring, which is widely advocated for inclusion in listening and lecturing training. We analysed a corpus of 46 engineering lectures from Italy, Malaysia, New Zealand and the UK. Comparing the EMI with the L1 corpus revealed that importance marking happened to the same extent. However, this belies substantial frequency differences between the two components that make up each of these corpora. We further found that both L1 and EMI lecturers used a large variety of markers and lexis. Overwhelmingly, differences could not be attributed to the different lecture contexts. Instead, they were typically due to idiolect. This suggests the specificity of language use in lectures and the dangers in lumping together varieties as representing EMI or L1 lecture discourse. We conclude with pedagogical implications for lecturer training.