Both historians and psychiatrists have tried to explain the recurrent attacks of mental and physical illness experienced by King George III of Great Britain. Although the porphyria hypothesis is widely accepted, this diagnosis assumes that the king’s breakdowns were not precipitated by extreme stress. This assumption was tested using single-case historiometric methods. Biographical data were compiled to form two extensive chronologies of the monarch’s life, one for stressful events and the other for pathological symptoms. From this information 22 independent judges reliably assessed fluctuations in stress (total, personal, and political) and health (total, physical, and mental) across 624 consecutive months between 1760 and 1811. The cross-correlations were then calculated for the raw, first-differenced, and prewhitened time series. A consistent tendency appeared for the king’s health to deteriorate after increases in stress, most frequently with a 9-month delay. The current study demonstrates the utility of applying quantitative techniques to a psychobiographical debate hitherto examined solely by qualitative approaches.