Polio provocation has concerned health professionals for nearly a century. Before an effective polio vaccine was licensed in 1955, evidence that certain paediatric injections could precipitate a polio infection and severe forms of paralysis informed medical debates, experiments and shifts in public health policy. This article explores how the theory was received and approached in the United States and the consequences of its protracted resolution. It contends that although medical professionals sought to maximise health benefits for American citizens, varying conceptions of what constituted an appropriate balance of risk inspired diverse health policy outcomes.