It is often claimed that the condition now known as Bipolar Affective Disorder is particularly common among creative personalities. Quite a number of classical composers have been said to have suffered from it. But inevitably posthumous diagnoses tend to be rather vague. So which of the classical greats look most likely to have been bipolar? And can their condition be related in any meaningful way to the music they composed? Stephen Johnson (himself a sufferer from bipolar disorder) investigates.