BACKGROUND/OBJECTIVE: To test the feasibility of a mixed-phase, pilot, placebo-controlled, double-blind trial of mistletoe therapy (MT) with an embedded qualitative study in the UK National Health Service (NHS) setting. METHODS: The aim was to recruit 45 patients via an NHS oncology centre with a diagnosis of early or locally advanced breast cancer. Participants were allocated to Iscador® Malus, Iscador® Pinus, or physiological saline (placebo). Diaries and quality-of-life questionnaires were administered. Qualitative interviews were conducted with participants, oncologists, and nurses. Feasibility was assessed by recruitment, retention, adherence, blinding, and safety. RESULTS: Sixty-seven patients were approached between August 2019 and March 2020, 15 gave consent, 14 participants were randomised, and 2 withdrew during the trial. Ten participants and five staff were interviewed. Barriers to recruitment were the additional treatments/time, extra injections, and the possibility of placebo allocation. Adherence was very good whilst the participants were on the study therapy. Diaries and interviews indicated that 11/14 participants struggled with injections and skin reactions. There were 22 adverse events due to the MT, related to the injections or skin reactions. CONCLUSION: This pilot study examined the feasibility of conducting a randomised placebo-controlled, double-blind trial of mistletoe therapy for breast cancer patients within the UK NHS. The results describe the challenges and achievements of recruitment, retention, adherence, blinding, and safety in this context.