Anticipation is a fundamental aspect of social life and, following Weber, the hallmark of social action—it means trying to take others’ responses to our actions into account when acting. In this article, we propose and argue the relevance of anticipation to the sociological study of diagnosis. To that end, we introduce and elaborate on the concept of diagnosing by anticipation. To diagnose by anticipation is to consider diagnoses as cultural objects imbued with meaning, to anticipate how others will respond to their meaning in situ and to adapt the choice of diagnosis to secure a desired outcome. Unlike prognosis, which seeks to predict the development of a disease, diagnosing by anticipation entails seeking to predict the development of a case and the effect of different diagnostic categories on its trajectory. Analytically, diagnosing by anticipation therefore involves a shift in diagnostic footing, from trying to identify what the case is a case of, to trying to identify which diagnosis will yield the desired case trajectory. This shift also implies a stronger focus on the mundane organisational work of operating diagnostic systems and coordinating case trajectories within and across social systems, to the benefit of the sociology of diagnosis.