Digital patient monitoring gains importance for quality of clinical cancer care. Our case report provides insight into usability and acceptance of a smartphone app for monitoring of electronically captured patient-reported outcomes in patients undergoing immunotherapy. During 3 months, 6 patients with advanced or metastatic PD-L1-positive cancer of the lung, prostate, and bladder who underwent checkpoint immunotherapy were using the Consilium app for standardized and structured electronic reporting of symptoms and therapy side effects. We evaluated the number and quality of symptom entries as well as usability and safety of shared reporting between the patient and the treating physician. Duration of anti-PD-L1-directed immunotherapy in the 6 patients ranged from 4 to 10 months and comprised a total of 21 anti-PD-L1-directed immunotherapy cycles. Patients reported between 4 and 16 different symptoms, of which the most frequent (57%) were dry cough, fatigue, shortness of breath, fever, and appetite loss. Overall, 1,279 symptom entries were counted, corresponding to 2.4 symptom entries per patient per day. Symptom severity grading ranged from 0.1 (very slight symptoms) to 7.8 (severe symptoms), which triggered prespecified alerts in 4 of the 6 patients. No unplanned visits were noted, and no safety issues occurred. Satisfaction with the app usability was high, as was the beneficial effect on consultation. Usability and reviewed data entries indicate high shared reporting efforts of patients and treating physicians and overall satisfaction with electronically reported patient outcomes.