The concept of “a motility psychosis” was introduced by Wernicke at the turn of the century in opposition to Kraepelin's “two entities principle.” The term cycloid psychosis was utilized by Kleist in 1926 for this group, which was further subdivided into “anxiety-elation,” “confusion,” and “motility” psychoses by Leonhard. This diagnostic concept is presently used in Germany and Scandinavia, but two recent British investigations have shown that similar patients occur in London, where they comprise 8 to 10 per cent of admissions for acute psychosis. The diagnosis depends on the presence of “confusion” or a polymorphic disease picture with a mixture of affective and schizophrenic symptoms, often rapidly changing from one to the other. Some of these patients would be diagnosed as mania, others as schizophrenia in Britain and as schizoaffective or mood-incongruent affective disorder in the United States. Follow-up of the London patients showed that the cycloids had an exceptionally good prognosis—much better than schizophrenia, and at least as good as manic-depressive patients, but their clinical picture did not correspond to the group of patients diagnosed as suffering from bipolar affective disorder. However, there is some evidence to suggest a relationship between cycloid and manic-depressive disorders, including the beneficial effect of electroconvulsive therapy (ECT) and lithium in both the cycloid patients and the bipolar affective disorder patients. American psychiatry has recently decided to adopt a very strict view of schizophrenia, limiting the diagnosis to patients with hebephrenia or chronic paranoid hallucinatory psychoses. This will leave a large group of patients previously considered to be suffering from “acute schizophrenia” who fit very awkwardly into the category of the affective disorders. It is from this group that cycloid patients are recruited so that the concept may enable psychiatrists to collect and study a fairly homogeneous group which was previously scattered among several widely different diagnostic classes.