A heterogeneous group of disorders characterized by the acute onset of psychotic symptoms such as delusions, hallucinations, and perceptual disturbances, and by the severe disruption of ordinary behaviour. Acute onset is defined as a crescendo development of a clearly abnormal clinical picture in about two weeks or less. For these disorders there is no evidence of organic causation. Perplexity and puzzlement are often present but disorientation for time, place and person is not persistent or severe enough to justify a diagnosis of organically caused delirium. Complete recovery usually occurs within a few months, often within a few weeks or even days. If the disorder persists, a change in classification will be necessary. The disorder may or may not be associated with acute stress, defined as usually stressful events preceding the onset by one to two weeks.