This article reports a study on metaphor comprehension by the international students whose first language is not English, while attending undergraduate lectures at a British university. Study participants identified words or multiword items that they found difficult in extracts from four academic lectures, and they interpreted metaphors from those extracts. Among the items reported as difficult, we established the proportion of metaphorical items, plus the proportion of items composed only of words familiar to the students. We developed a measure of the extent of students' awareness of their metaphor interpretation difficulties, plus a scheme for categorizing the most common types of metaphor misinterpretations. We found that, of the items that were difficult though composed of familiar words, [approximately]40 per cent involved metaphor. Further, when the students misinterpreted metaphors, they only seemed aware of having difficulty in [approximately]4 per cent of cases. As university lecturers use metaphors for important functions, such as explaining and evaluating, such international students may thus be missing valuable learning opportunities. Our error categorization scheme could be used in helping English learners with metaphor comprehension.