Background: The benefits of electronic patient reported outcomes (PRO) questionnaires have been demonstrated in many settings, including in hospitals and patient homes. However, it remains to be investigated how melanoma patients and their treating clinicians experience the electronic self-reporting of side effects and the derived communication. Objective: The primary objective of this study was to examine patients' and clinicians' experiences with an eHealth intervention for weekly monitoring of side effects during treatment with immunotherapy. Methods: An eHealth intervention based on questions from the PRO-Common Terminology Criteria for Adverse Events (CTCAE) library was used and tested in a randomized clinical trial with patients receiving immunotherapy for malignant melanoma and clinicians at a university hospital in Denmark. On a weekly basis, patients reported their symptoms from home during the treatment via a provided tablet. The electronic patient reports were available to clinicians in the outpatient clinic. A mixed methods approach was applied to investigate the patients' and clinicians' experiences with the intervention. Data from patient experiences were collected in a short survey, the Patient Feedback Form. Moreover, a subset of the patients participating in the survey was interviewed about their experience. Furthermore, one focus group interview with clinicians was carried out to elucidate their views. Results: A total of 57 patients completed the Patient Feedback Form, and 14 patients were interviewed. The focus group interview included 5 clinicians. Overall, patients and clinicians were satisfied with the tool. They believed it enhanced patients' awareness of side effects and increased their feeling of involvement. The patients reported that it was easy to fill out the questionnaire and that it made sense to do so. However, a minority of the patients expressed in the interviews that they did not believe that the health care professionals had seen their reports when they came to the clinic, and that the reporting did not lead to increased contact with the department. Conclusions: Overall, satisfaction with the eHealth intervention was high among patients and their treating clinicians. The tool was easy to use and contributed to greater symptom awareness and patient involvement. Thus, in terms of patient and clinician satisfaction with the tool, it makes sense to continue using the tool beyond the project period.