This study investigated factors influencing teachers' intention and willingness to teach about cancer in the context of the theory of planned behaviour. Because teaching about cancer is mandatory in Germany, we compared n=355 teachers' intention to teach about cancer under mandatory conditions and their willingness to teach about cancer if they had free choice. Teachers signalled strong intention and a strong willingness to teach about cancer. Teachers did not perceive teaching about cancer to be a burden and strongly considered it to be necessary. Structural equation modelling showed that perceived autonomy was the only significant predictor of teachers' intention, whereas attitude towards the perceived necessity to teach about cancer, attitude towards the perceived burden of teaching about cancer and social norm were significant predictors of willingness. We attributed these differences to the observation of a ceiling effect for intention but not for willingness. There were systematic differences among the variables predicting teachers' intention and willingness depending on whether the teachers had personal experience with cancer. Indirect measures yielded insights into which teacher beliefs affected the direct measures. We discuss implications for professional development, such as reducing the burden of teaching about cancer by adequately addressing emotionally-charged classroom situations.