By analyzing two of the "consilia" written by Paolo Zacchia, this paper explores what constituted a medico-legal fact in the law-courts of 17th century Rome. A founding father of legal medicine, Zacchia was strongly committed to strengthening the physicians' authority on a growing range of issues. A key element in his "consilia" was the identification of the "series facti", the sequence of events which he extracted from the legal dossier and assessed in order to establish its correct medical chronology. To do this he would mobilize his doctrine, but also engage with the accounts of the events which had already been produced in the courtroom, including witnesses' testimonies. The paper discusses Zacchia's practice in relation to the history of the genre of "consilia" in medical and legal writing, and in connection with contemporary juridical debates on the competence of witnesses, both lay and expert, in establishing facts.