In 1969 it was proposed that the episodic madness suffered by King George III (1738–1820) resulted from an acute hereditary porphyria, variegate porphyria, caused by deficiency of protoporphyrinogen oxidase. The diagnosis was based on the historical archive and a contentious claim that living members of the House of Hanover were affected with the condition. A re-examination of the medical evidence and the appearance of new historical material have suggested that porphyria did indeed exist in the Royal Houses of Europe. We report the analysis of hair obtained from George III. Although no genomic DNA could be obtained, metal analysis revealed high concentrations of arsenic. Since arsenic interferes with haem metabolism, it might have contributed to the King's unusually severe and prolonged bouts of illness. We have identified sources of arsenic in the context of the medication George III received from physicians.