George III of England's episodic psychotic illness is described. The data has previously been used to substantiate the diagnosis of an unlikely hereditary metabolic disease, porphyria. A scientific perspective, aimed at removing psychiatric diagnosis from dependence on subjective evaluations, is corrected by the diagnosis of a mood disorder. His unipolar mania of late onset is a syndrome, the sine qua non of bipolar disorder, in this instance complicated by the toxicity of quinine, antimony, and purgatives.