Doctors occasionally make diagnoses in strangers outside of formal medical settings by using the medical skill of visual inspection, such as noticing signs of melanoma or the symptoms of hyperthyroidism. This may cause considerable moral unease and doubts on the side of the diagnosing physician. Such encounters force physicians to consider whether or not to intervene by introducing themselves to the stranger and offering an unsolicited medical opinion despite the absence of a formal doctor-patient relationship. A small body of literature (...) has addressed the topic of the unsolicited medical opinion, often with a primary focus on practical advice. This article seeks to establish an ethical-theoretical basis for physicians' ethical obligation to offer an unsolicited medical opinion when they make a diagnosis by visual inspection in a stranger outside of the formal medical context. Using a utilitarian approach, it is argued that, if it is in the physicians' power to prevent a possible loss of well-being, without thereby sacrificing anything of equal value, physicians have an ethical obligation to intervene. (shrink)