During 1989 and 1990 the United Kingdom National Health Service have used the object- oriented analysis technique 'Ptech' in a number of projects to develop a specification for clinical health systems. It was found that it was possible and necessary to produce an abstract model which should be of general use to all specialties and is expected to underlie future developments for clinical systems in the UK when incorporated into the wider Common Basic Specification, a conceptual model encompassing the whole of health care. This paper gives a brief description of Ptech and notes the principal lessons that have been learned during the projects. Ptech uses a structural view which describes classes and the relationships between them linked to a behavioural view which focuses on events. The event networks together with the approach of dynamic classification (objects changing class) are significant extensions to the object-oriented paradigm. The projects demonstrate that the technique enables users to be analysts, giving the user community a more direct control over development. The analysis technique proved useful in introducing object-oriented concepts whilst retaining the choice of an object-oriented or more traditional implementation.