We investigated Iranian secondary-school English teachers' interpersonal behaviour with a validated culturally-adaptive Iranian version of the Questionnaire on Teacher Interaction. Data were collected from 971 Iranian secondary-school students (398 students participated in the pilot study and 573 students in the main study) and 55 Iranian secondary-school English teachers (23 teachers participated in the pilot study and 32 in the main study). The construction of the instrument started with an open-ended questionnaire followed by semi-structured interviews to prepare items for the questionnaire. Next, expert judgement, pilot administration of the questionnaire and statistical analyses ensured its reliability and construct validity before final administration of the questionnaire to the main participants followed. The study supported the validity and reliability of the questionnaire and confirmed its usability in the Iranian secondary-school context. Furthermore, the results indicated that the students perceived their English teachers as being Tolerant and Authoritative, showing higher degrees of friendship, leadership and understanding behaviour and relatively low levels of uncertain and dissatisfied behaviour. The results showed a discrepancy between students' perceptions of actual and ideal teacher interpersonal behaviour. Also students' perceptions of their actual teachers' interpersonal behaviour were significantly different from those of the teachers' self-perceptions. One important implication of this study is that the QTI can be used in secondary-school contexts as a valuable feedback instrument to encourage teachers to reflect on their performance and improve their interpersonal behaviour as a part of their professional development.