Health care has an increasingly demand for mobile applications (App), but studies are rare, which explore the added value and benefits for patient and physician. Patients in different disease groups or physicians from different specialties are likely to have different demands. Research should focus on selected groups to better understand their individual demands. Our study intends to identify the added value of mobile symptom tracking in a selected subgroup of patients. We designed an App for breast cancer patients, who receive ambulant chemotherapy in a breast center. The patients track regularly their well-being and adverse events (AE) with the smartphone- or web-app and share it with the physician in the medical consultation. The data entry was designed to meet patient needs based on previous usability testing.