Background. Classification of psychosis lacks a biological basis and current diagnostic categories may obscure underlying continuities. Data reduction methods of symptom profiles within a population-based cohort of people with a wide range of affective and non-affective psychoses may permit an empirical classification of psychosis.

Method. OPCRIT (operational criteria) analysis was performed on 387 adults aged 18–65 years in an attempted ascertainment of all patients with psychosis from a geographical area with a stable population. The data were analysed firstly using principal components analysis with varimax rotation to identify factors, and secondly to establish latent classes. Information relating to key variables known to be of relevance in schizophrenia was coded blind to the establishment of the classes and dimensions.

Results. Striking correspondence was obtained between the two methods. The four dimensions emerging were labelled ‘depression’, ‘reality distortion’, ‘mania’ and ‘disorganization’. Latent classes identified were ‘depression’, ‘bipolar’, ‘reality distortion/depression’ and ‘disorganization’. The latent classes corresponded well with DSM-III-R diagnoses, but also revealed groupings usually obscured by diagnostic boundaries. The latent classes differed on gender ratio, fertility, age of onset and self-harming behaviour, but not on substance misuse or season of birth.

Conclusions. Both dimensional and categorical approaches are useful in tapping the latent constructs underlying psychosis. Broad agreement with other similar studies suggests such findings could represent discrete pathological conditions. The four classes described appear meaningful, and suggest that the term non-affective psychosis should be reserved for the disorganization class, which represents only a subgroup of those with schizophrenia.