Traditional descriptions of delusions have emphasized the conviction with which they are held and their resistance to change. This study utilizes data from a large cohort of delusional subjects to assess the persistence and stability of delusional beliefs, and the predictors of change. Data were collected from 1,136 acutely hospitalized psychiatric patients, reinterviewed at 10-week intervals for 1 year. Persistence of delusional beliefs was determined for those delusional subjects with at least one follow-up visit (n = 405), and stability for the subset with delusions at two or more points in time (n = 262). Marked plasticity in delusional beliefs was observed, with one third of delusional subjects at any interview no longer delusional 10 weeks later. Persistence of delusions was associated with schizophrenia, global psychopathology, and having acted on a delusion, among other variables. Most subjects showed variation in the content of their primary delusion over time. Delusions appear to be more fluid over relatively short periods of time than has been suggested by many classic descriptions and contemporary formulations.