OBJECTIVE This study investigated acute and nonacute brief psychoses. On the basis of previous work, the authors proposed that 1) acute brief psychoses occur predominantly in females, 2) they often do not conform to the diagnoses of DSM-III-R, 3) they are temporally stable, and 4) nonacute brief psychoses do not share these distinctive features. METHODS The data are from a follow-up study of 221 first-admission patients with affective and nonaffective psychoses. Patients were given extensive assessments at initial evaluation, 6-month follow-up, and 24-month follow-up. The research team made consensus ratings of the presence of psychosis, DSM-III-R diagnosis, mode of onset of disorder, and course of disorder. Brief psychoses were defined by a diagnosis of nonaffective psychosis at the initial evaluation and a rating of full remission at 6-month follow-up; acute brief psychoses met the additional criterion of acute onset as defined by ICD-10. RESULTS Twenty (9%) of the 221 psychoses were brief psychoses. Only seven (3%) were acute brief psychoses, but among these, six occurred in women, five were undiagnosable, and none had evolved into an affective disorder or a chronic disorder by the time of the 24-month follow-up. The 13 nonacute brief psychoses did not exhibit distinctive features, and five of them later evolved into chronic disorders. CONCLUSIONS Acute brief psychoses emerged as a highly distinctive and temporally stable form of psychosis that may merit a separate diagnostic classification. The more numerous nonacute brief psychoses may represent mild forms of nonaffective psychoses such as schizophrenia.