This paper describes the information system for a cumulative psychiatric patient case register which has been in operation for ten years in a community with a population of 700,000 persons. The structure of the data files and the general philosophy behind them are explained. The problems of accuracy and of intra-file and inter-file record linkage by name and matching are discussed. It is concluded that well-trained, careful clerical help are as important as elaborate computer editing programs for maintaining the highest possible level of accuracy.