This study in the history of criminal law examines a homicide case from the eighteenth-century legal practice of the Free Royal City of Debrecen that has not yet been explored in detail. Drawing on archival documents relating to the criminal proceedings initiated before the city’s magistracy in 1768, the study reviews the main phases of the process, reconstructs the arguments of the prosecution and the defence, and analyses how the defendant’s criminal responsibility was assessed in light of evidence indicating that he suffered from a mental disorder. Beyond clarifying the significance of the defendant’s mental state from a criminal law perspective, the case also provides an opportunity to explore the contemporary concept of lucidum intervallum in greater depth and to highlight evidentiary challenges associated with proving it in eighteenth-century criminal proceedings.