The hair samples of Ferdinand II of Aragon (1467–1496), King of Naples, whose mummy is preserved in the Basilica of San Domenico Maggiore in Naples, showed a high content of mercury, with a value of 827ppm. Furthermore, examination using a stereomicroscope and a scanning electron microscope (SEM) of head and pubic hairs of Ferdinand II, revealed a lice infestation. The reasons for the massive presence of the mercury in the king's hair are discussed and contemporary literature regarding the use of this metal in medical therapies and in cosmetic practices is analysed. As a result, the high value of mercury in the hair of Ferdinand II can be attributed to antipediculosis therapy, applied as a topic medicament. This case represents an important finding for the history of medicine, because demonstrates that in the Renaissance mercury was applied locally not only to treat syphilis, as well attested by direct and indirect sources, but also to prevent or eliminate lice infestation.