This article discusses the trial of a woman accused of murder in 1890 whose defence rested on the claim that she acted unconsciously under the hypnotic influence of her older lover. This relatively banal case brought together two rival schools of French psychiatry – that of J.-M. Charcot in Paris and that of Hippolyte Bernheim in Nancy – and provided a wide-ranging examination of views on the nature of unconscious mental activity as well as the social, political and professional implications that their theories on hypnotism and hysteria contained. Discussions on women's sexuality, family relations, crowd behaviour and political radicalism all played a part in the debate and are examined through the case study that the trial of Gabrielle Bompard permits. Moreover, the trial shed incidental light on the campaign by physicians against amateur healers and hypnotists whom they blamed for unleashing a wave of mass hysteria through their theatrical representations. The episode was one important element in the struggle for the passage of the law of 30 November 1892, which outlawed amateur practitioners and established the medical monopoly over healing in France.