The intention of this paper is to give an overall view of. (a) The part a computer-based hospital information system (HIS) might play in a hospital. (b) The consequences of this part as to requirements for the contents (components, structure) of a hospital information system. This intention takes shape as follows: (1) Characteristics of data and their use in a hospital are considered including sensitivity, poor standardisation, high availability and long term storage required, incompleteness. (2) An outline is given of which part a HIS might play in this context and how a HIS can achieve this. Quantitative as well as qualitative improvements are described to be realised by checking the input, fast accessibility, improved standardisation, improved exchange of data, improvement of coordination of activities in the hospital. The concept of integration is shortly described. The logical structure of most HISs is introduced: a central databank with a huge storage capacity, accessible for authorised users by means of various application-packages (software) and a large number of terminals (hardware). (3) As an example of an operational HIS, the Leiden University Hospital HIS is described. The following subjects are covered: technical form (hardware, availability for the users), organisation, the applications, the use of the system, costs and benefits.(ABSTRACT TRUNCATED AT 250 WORDS)