Charles Singer’s retrospective diagnosis of Hildegard of Bingen as a migraine sufferer, first made in 1913, has become commonly accepted. This article uses Hildegard as a case study to shift our focus from a polarised debate about the merits or otherwise of retrospective diagnosis, to examine instead what happens when diagnoses take on lives of their own. It argues that simply championing or rejecting retrospective diagnosis is not enough; that we need instead to appreciate how, at the moment of creation, a diagnosis reflects the significance of particular medical signs and theories in historical context and how, when and why such diagnoses can come to do meaningful work when subsequently mobilised as scientific ‘fact’. This article first traces the emergence of a new formulation of migraine in the nineteenth century, then shows how this context enabled Singer to retrospectively diagnose Hildegard’s migraine and finally examines some of the ways in which this idea has gained popular and academic currency in the second half of the twentieth century. The case of Hildegard’s migraine reminds us of the need to historicise scientific evidence just as rigorously as we historicise our other material and it exposes the cumulative methodological problems that can occur when historians use science, and scientists use history on a casual basis.