Cognitive-behavioural models of psychosis have been widely discussed in the literature, with a focus on the beliefs about the voices that are a component of auditory hallucinations. This study examines the ability of beliefs about the voices to predict the course of psychiatric symptomatology in psychosis over a 1-month period, compared with changes in depressive symptomatology. Thirty people with schizophrenia or schizo-affective disorder participated in this study. Their mental state functioning, depression, and beliefs about their voices were assessed in interview. Mental state functioning and depression were assessed again after 1 month, in addition to antipsychotic medication compliance over the previous month (rated by self and the psychiatrist). The strongest predictor of a worsening of psychiatric symptoms over the month was increased depression in the previous month. Using p < 0.1 criteria to offset possible Type II errors in the small sample, a person's belief that their voices were very powerful was also predictive of psychiatric deterioration. The ways in which beliefs about the power of the voices can be challenged in cognitive-behaviour therapy with psychosis is discussed, as are directions for future research.