Phencyclidine use has been noted to produce a psychosis of several week's duration in a small fraction of users. Descriptions of the premorbid personalities of those who became psychotic resemble descriptions of LSD and marijuana users who experienced prolonged psychiatric difficulty. In addition, the psychosis produced can often be recognized as a "hallucinogen" psychosis. Certain features of the phencyclidine psychosis, namely the neurologic abnormalities, dose-related severity of symptoms, and regularity of the length of illness, are not noted with other psychedelic drugs, leading to the conclusion that PCP psychosis is a drug effect rather than a brief functional psychosis precipitated by the disintegrating PCP experience. However, the infrequent occurrence of psychosis in the (apparently) large exposed population still suggests that this is a combination of drug effect and vulnerable, pathologic personality.