Shakespeare's plays, and in particular King Lear, have been a favorite source of clinical observation and diagnosis for psychiatrists for the past two centuries. Most authors agree that the description of Lear's mental symptoms is remarkably consistent and close to life. This article summarizes previous attempts to diagnose the mental illness of King Lear, featuring, among others, such entities as mania, senile dementia, delirium, depression, and brief reactive psychosis, and offers a new diagnosis according to the modern diagnostic criteria, namely, bipolar I disorder, most recent episode manic, severe with psychotic features.