Utilising a rigorous experimental trial, significant increases in teacher implementation of the HIV Supplement to the Western Australian Health Education K-10 Syllabus amongst an intervention group was observed, compared with a comparison group of teachers. This 12-month intervention trial involved 124 health education teachers and 2541 students from 36 metropolitan government senior high schools in Western Australia. The intervention had a minimal effect on changing HIV- related knowledge and attitudes of intervention teachers. However, teachers who were in the intervention group, who had a personal copy of the HIV Supplement, and attended the intervention training, were those most likely to implement the HIV Supplement activities. In addition, intervention teachers who had previously attended an sexuality in- service training in the previous three years, and who had increased their belief that their peers perceived them as innovative, were more likely to implement at least one HIV Supplement core activity. It is acknowledged that difficulties in assessing alternative HIV education lessons that teachers may have implemented, due primarily to the low response rate associated with the teacher logbook prevented greater understanding of the observed low implementation rates to the HIV Supplement core activities. However, the results from this study suggest that single measures of implementation are limited in their ability to measure teacher implementation, and that multiple measures used to form a implementation index may provide a more valid measure of teacher implementation. The results further suggest that health education teachers who have positive attitudes towards HIV education, and who are comfortable and knowledgeable with HIV-related materials, may teach using existing HIV-related materials, rather than the HIV Supplement activities. Finally, this study concluded that ensuring teachers have a personal copy of the HIV Supplement, and that they attend an in-service training with well-specified implementation tasks, is likely to result in greater implementation of the HIV Supplement.