Objective - The aim of our research is to evaluate the prevalence of psychiatric illnesses by means of data collected at the Florence municipality community mental health services encompassing all hospital and extramural activities, providing the same kind of information given by Psychiatric Case Register (PCR) covered areas. Design - It has been carried out a one week prevalence assessment through a cross-sectional analysis of clinical records gathered by psychiatric services of the submitted area. Population studied - The inquiry has considered the area of USL (Administative Local Authority of National Health Service) 10/A, 10/B, 10/C and 10/D, which constituite togheter with USL 10/E, the Florence municipality area. Results - Total and specific rates for diagnostic categories, sex and age reveal a large variability among the four considered USLs confirmed by standardized rates: the 10/A and 10/B USLs, that include the historic centre of Florence, present the highest standardized rates (10/A 207/100.000, 10/B 189/100.000), especially for the most severe disorders (Schizophrenia: 10/A 110/100.000, 10/B 87/100.000; Major Affective Disorders: 10/A 32/100.000, 10/B 38/100.000; Organic Psichosys: 10/A 6/100.000, 10/B 9/100.000). Conclusion - In the present study, the prevalence rate for all diagnoses for the whole area considered, is about 150.6/100.000, quite close to the estimate calculated in Verona-Sud (200/100.000). As to the different rates among USLs we have to bear in mind the development of the delivery care system, and the available clinical settings (on day hospital and residentials activities for long-term patients).