The present study explored the developmental changes of teacher-student interpersonal relationships as well as that of academic motivation among first-grade secondary school students. In addition, the link between teacher-student interpersonal behaviour and academic motivation across the school year was investigated. The data were collected 5 times within a school year, from 566 students of 20 mathematics and English classes, from 3 secondary schools in The Netherlands. Multilevel growth curve modelling was applied. Analysis of within-year changes in teacher-student interpersonal relationships revealed that the quality of relationships decreased over time. The decrease was more pronounced for Proximity than for Influence. Moreover, students' controlled motivation increased slightly, while autonomous motivation decreased systematically over time. Teacher-student interpersonal relationships are significant predictors of autonomous motivation. Several determinants like subject taught, class type, teacher gender, and student gender can explain differences in developmental trajectories of both interpersonal behaviour and academic motivation over time. (Contains 7 figures, 10 tables and 2 notes.)