The following paper analyses the transition from a host centered to a client-server based Hospital Information System (HIS) which takes place at the University Hospital of Gießen, Germany, since 1995. It focuses on the overall concept and practical realisation of the cornerstone within this transition, the Macintosh Mosaic Software Architecture (MMSA) which delivers now more than 90% of the clinical data which is entered on the wards and within the operating rooms. Besides the technical problems of the project, the paper also describes some aspects of the organisational challenges which took place within a typical German University Hospital during the last 4 Years and which mainly affect the daily management of a large HIS.