Mental disorder contributes heavily to the total burden of disability in the population, more especially in the younger age groups. This international study, coordinated by WHO concerns a cohort of new patients coming from a geographically circumscribed area, and suffering from a functional psychosis of non-affective type. Results show a rather low annual administrative incidence-rate, compared with those found in Britain, Iceland and Sweden. The very low incidente of schizophrenia (ICD 295), with little difference between the sexes, is striking. Mental health statistics of age-specific admission and prevalence rates per 10,000 of the population indicate however that the significante of this group of diseases lies in what happens next to those affected. The planned follow-up of the cohort will throw more light on the evolution of this type of psychosis, perhaps resulting in a retrospective increase in the initially low incidence-rate for schizophrenia in particular.