Objective: The short-term diagnostic stability of schizophrenic and other psychotic disorders was examined in first-admission patients, with attention to the principal reasons for diagnostic change. Method: Hospitalized first- admission patients (N=278) participating in an epidemiologic study were interviewed at baseline and after 6 months with the Structured Clinical Interview for DSM-III-R. A best estimate diagnosis was made at both time points with the use of all available sources of information. Reasons for changes in diagnosis were determined by two psychiatrists. Results: Affective psychosis and schizophrenic disorders were relatively stable broad diagnostic categories over the 6-month period, with 86.5%-88.9% of the patients remaining in the same category, although findings for specific diagnoses within these categories ranged from 61.5% to 85.7%. The groups with unknown and nonspecific diagnoses showed less stability; the diagnoses of more than one-third of these patients remained unknown or nonspecific at the 6-month evaluation. If the 6-month diagnoses are used as the research standard, somewhat lower percentages of patients received the same diagnoses at baseline. Forty-three percent of the changes in diagnosis were attributed to the clinical course of illness; the rest were attributed to the diagnostic process itself. Conclusions: A longitudinal diagnostic assessment based on multiple sources of information is crucial for categorizing first-admission psychotic patients, particularly those who do not initially fit into a DSM- III-R category. The short-term stability of a diagnosis is a function of multiple factors, including the changing clinical picture, additional sources of information, and new interpretations of original data.