Manic-depressive psychosis is characterized by severe swings of mood, which may be depressive, hypomanic or manic in type: occasionally a circular form is seen, with successive mood fluctuations in both directions in the same individual. Kraepelin indicated the frequency of these various types (depression alone 48·9 per cent, mania alone 16 6 per cent, circular 34·5 per cent), but also emphasized how difficult it is to predict the precise course of the disease in any individual patient. He stated: ‘we cannot speak of even an approximate regularity in the course of the disease’, and ‘the kind and direction of the attack and the intervals do not by any means remain the same in the individual case’ (Kraepelin, 1921).