Although it is commonly said that the sanctuary of Lourdes was built on the fame of its miracles and numerous recorded declarations of healing, this article intends to show that it was also built on a requirement of truth. Deployed in a variety of domains (in the medical diagnoses of healing as much as historical accounts of specific events), this requirement of truth is manifest in investigations that take a variety of forms. A better understanding of these enquiries is important, because they reveal a trick of the religious institution that, while remaining in the background, lets certain specific forms of rationality play themselves out in order to foil critics as well as to strengthen its position and discourse. The article consequently examines how this desire for truth in the history of events at Lourdes manifests itself, what target it choses, who is calling for it, and above all, who is thought capable of establishing it.