Introduction : The idea of therapeutic fasting with healing virtues is circulating among cancer patients. Our study aims to improve knowledge of this practice, which is contraindicated in France during chemotherapy, and to establish recommendations to facilitate exchanges between doctors and patients.Methods : Chemotherapy patients completed a self-questionnaire on diet, therapeutic fasting and alternative medicine. A subsample of patients intending to follow dietary restrictions were interviewed.Results : Among the 133 participants, more than half had changed their diet and/or had heard of therapeutic fasting. Twenty-one patients intended to fast or have dietary restrictions during chemotherapy. These were mainly women, with an average age of 56 years, being treated for breast cancer, using alternative medications. They had little interaction with the health care team but would have liked to have had some with their oncologist. Nine patients were interviewed. They had tested short fasting and/or a ketogenic diet to improve treatment efficacy, reduce side effects and/or gain more control over their management. They did not dare to talk about it with the oncologist but regretted their silence. They are often advised by naturopaths and have tested homeopathy to accompany their treatment.Conclusions : Patients explain that they want to put all the odds in their favor. They would like the medical profession to offer times for discussion on fasting, silence being perceived as potentially harmful.