Dividing the delivery system of larger psychiatric hospitals into defined smaller catchment areas called sectors, implies a contribution to quality assurance. Continuous monitoring and evaluation of a period before and after this reorganisation, which took place in 1994 for the central region of Zürich (Psychiatric University Hospital), allows perception of expected and possible unexpected changes. All newly registered patients were recorded within a time span divided into five periods of six month five each, the last one covering the first six months after sectorisation. This design permits to answer the question as to how fast the intended changes after sectorisation occur, and also to control side effects. Results show that sectorisation resulted in a marked increase of registrations from the defined catchment area of each sector, and that the investigated institutions achieve attainment of the aim to effect a special selection of psychiatric patients characterised by considerable psycho-social deficits. The two sectors display differences in respect of socio-demographic characteristics of newly registered patients.