This paper describes a computer data handling system which includes a powerful interactive program for the presentation and rapid modification of graphs. The system was developed in a dialysis and transplantation center in its early phases and is now in use in 20 renal centers in Europe and North America. Recent developments are a response to the varying demands of different centers. Our work describes some of the wide range of functions for which the system is used and the favorable response of staff working with a system that is easy to use and flexible, and which is also responsive to the requirements of the clinical user without the intervention of a computer programmer. The versatility of the system has made it possible to transfer it to other clinical specialties. As well as retaining its primary functions of improving the day-to-day care of patients and increasing the understanding that doctors have about their patients, the computer is used for research and administrative functions.