OBJECTIVE: Reports of increased rates of psychosis in prisons could be due to sampling and ascertainment differences. The authors compared two samples of subjects 16-64 years of age: those from the general population of residents in Great Britain and prisoners in England and Wales. METHOD: A random sample of remanded and sentenced male and female prisoners (N=3,142) and a two-phase, cross-sectional random sample of household residents (N=10,108) were assessed with structured questionnaires and the semistructured Schedules for Clinical Assessment in Neuropsychiatry. RESULTS: The weighted prevalence of probable functional psychosis in the past year was 4.5 per thousand (95% CI=3.1 to 5.8) in the household survey. In the prison survey, the weighted prevalence was over 10 times greater: 52 per thousand (95% CI=45 to 60). One in four prisoners with a psychotic disorder had psychotic symptoms attributed to toxic or withdrawal effects of psychoactive substances. The proportion of subjects with specific types of hallucinations or delusions did not differ between prison and household psychosis cases. CONCLUSIONS: This large study using standardized comparisons showed that the prevalence of psychosis in prisons is substantially higher than in the community and is deserving of greater attention to treatment and prevention. Apart from a minority of prisoners with symptoms attributable to psychoactive substances, the clinical symptom profile of psychosis is the same in both settings. Longitudinal research is needed to better understand these prevalence differences.