This article reassesses the role of gender in early modern demonic possession from a medical perspective. It takes as its starting point the demoniac Richard Mainy, who in 1585 claimed to be suffering from hysteria. Best known for its influence on Shakespeare's King Lear, Mainy's gender-crossing diagnosis should be read in the context of the close historical relationship between hysteria and epilepsy. While medical historians have viewed hysteria as the key possession-related illness, epilepsy was equally important. Both were seen as convulsive illnesses caused by an excess of reproductive fluids. Emphasizing the similarities rather than the differences between male and female sexuality, this shared etiology underpinned medical approaches to demonic possession.