Background: There is increasing interest in using e-health systems for symptom monitoring and alert triggering. ZEMY, a new software medical device, was developed to improve symptom management and enhance patient and health care team interactions. Methods: This was a three-month open-label, interventional, single arm study conducted at five French sites. Adult women with breast cancer (BC) initiating oral and/or parenteral cancer treatment received a smartphone with ZEMY installed and verbal training. Patients started cancer treatment on Day 1 and were followed for three months. Patients entered data for ten prespecified symptoms. The ZEMY software made recommendations to patients on the self-management of symptoms and transmitted automatic messages to the health care team. Primary outcome was patient feasibility response at 3 months (completion of ≥3 symptom reports and a report completion rate of ≥60% per patient). Secondary objectives included ZEMY symptom management, usability and satisfaction, and device deficiencies (DDs). Results: Overall, 54 patients were enrolled (Jun 2018 to Jan 2019) and 52 (96.3%) completed the study. Thirty-one (57.4%) patients were responders; this was not significantly higher than the predefined cut-off and ZEMY feasibility was not demonstrated. Forty-seven (87.0%) patients had ≥3 completed symptom reports and 33 (66.0%) patients had a report completion rate of ≥60% (missing: n = 4). In subgroup analyses, 22/30 (73.3%) patients with locally advanced BC and 24/36 (66.7%) patients aged 40–60 years were responders. ZEMY patient recommendations and health care team automatic messages were considered relevant at least once in > 74% of cases for five of the six symptoms that were reported at least once by more than 1/3 of patients (diarrhea, nausea, fatigue, cutaneous and mucosal toxicities, and anxiety/depression). Using a 0–10 visual analogue scale, mean ZEMY satisfaction score was higher for patients than investigators (6.3 ±2.9 vs 4.4 ±1.5). Overall, 95 DDs were reported in 37 (68.5%) patients. Main reasons for DDs were inappropriate recommendations (n = 51) and device malfunction (n = 36). Conclusions: The primary endpoint of the study was not reached; however, higher response rates were observed in subgroups versus the overall population, indicating that ZEMY may be useful in symptom management in these patients. Generally, patients had a favorable opinion of ZEMY. The DD rate observed indicates the need for better user support in the future. Clinical trial information: NCT03558490