Experiments showed a small computer could improve access to large concentrations of clinical data in renal dialysis and transplantation and enhance the clinical feedback loop which controls substitute renal function. Interactive access adds a sub-loop which allows close adaptation to clinical use. Prototype software was transferred to an LSI-11 microcomputer with 20 Mbyte cartridge discs, multiple remote VDUs and video graphic displays and printer-plotter. Data capture is both manual and by automatic transfer from laboratory computers. Push-button interaction displays clusters of data, including computed functions, as graphs and tables on variable time scales. programmed scans generate data-base analyses. The system reconfigures for individual sites. The microcomputer version has run routinely for a year and proved a practical tool at acceptable cost. Down-time has been small but strongly resented by medical and nursing staff. The system can accommodate 750 patients, but has also been 'stretched' for an oncology laboratory and configured for other specialities.