Recent studies have shown that the claim that King George III suffered from acute porphyria is seriously at fault. This article explores some of the causes of this misdiagnosis and the consequences of the misleading claims, also reporting on the nature of the king's recurrent mental illness according to computer diagnostics. In addition, techniques of cognitive archaeology are used to investigate the nature of the king's final decade of mental illness, which resulted in the appointment of the Prince of Wales as Prince Regent. The results of this analysis confirm that the king suffered from bipolar disorder type I, with a final decade of dementia, due, in part, to the neurotoxicity of his recurrent episodes of acute mania.