Recent research on medical communication discusses the role of argumentation in building physician-patient consensus to enhance shared decision-making. This paper focuses on the potential of using argumentation to establish the preliminary step of shared understanding of the diagnosis. This understanding is important in helping patients accept the disease and in increasing their involvement in care. The paper conducted an in-depth analysis of an observation of a medical encounter, triangulated with interviews with all participants, to illustrate how the lack of clear information and argumentation concerning the disease hindered the patient’s understanding and acceptance of it. This in turn led to difficulties in building a trusting relationship and in reaching treatment decisions. The paper discusses how using argumentation focused on the disease can allow a fruitful patient-centered discussion about the medical condition and treatment options. Keywords: Argumentation theory, shared decision-making, Patient-centered care, treatment decisions, breaking bad news