PURPOSE: In the Netherlands, online patient reporting of side effects is a new phenomenon. The aim of this study was to gain insight into patients' user experiences and the benefits of the web application BijKanker ('AlongsideCancer'). METHODS: Patients in seven hospitals were asked to make entries in BijKanker at least once a week. On logging in to the application for the first time (T1), patients were asked to complete a questionnaire on information needs and Internet use. Four months after starting their treatment (T2), they were asked to complete a second questionnaire on their experiences with BijKanker and its four functions: information; reporting side effects; communication with oncology nurses and clinicians; and data feedback. RESULTS: Ninety-nine patients logged in to BijKanker, 60 patients (61%) had completed the first questionnaire (T1) and 40 (40%) had also completed the second questionnaire (T2). In total, 1661 side effects were reported. Generally, patients experienced BijKanker as user-friendly and patients appreciated the attention given to their side effects. CONCLUSIONS: The user-centred design gives ample insight into user experiences and usability. The results provide useful starting points for improvements to the online application. We recommend to put much effort into supporting oncology nurses in the implementation of the application.