More than 5,700 women in Switzerland are annually diagnosed with an invasive tumour of the breast. A non-invasive (in-situ) breast cancer (BCIS) is less common than invasive breast cancer. Although these tumours are not life-threatening per se, they still need to be treated because about 20 – 25% of patients with BCIS are subsequently diagnosed with an invasive breast cancer. However, not much is known about how many women are affected in Switzerland and which factors influence the risk of invasive cancer after having had BCIS. We will use data of the cancer registry in the canton of Zurich (dating back to 1980), to examine the frequency of BCIS and the change in frequency over time until 2014. For these BCIS patients, we will analyse how they were treated, whether treatment changed in the past 35 years and how this affects a woman’s future risk of an invasive breast cancer diagnosis, and finally survival. This project aims to provide a better understanding of the prevalence of BCIS, the risk of a subsequent invasive breast cancer, and the effect of specific treatments. This information may ultimately help clinicians in their decisions regarding treatment of BCIS and the need for closer surveillance of specific BCIS patients.