The Department of Health is introducing a new system for collecting data about National Health Service (NHS) mental health care, the Mental Health Minimum Data Set (Glover, 2000). Individual records will be constructed about each period of care received by a patient in the specialist mental health services. In designing the system, a review was undertaken of how in-patient statistics, the only source currently collected in such detail, have been used. The aim was to identify features necessary to make the new source applicable in a similar manner. Here I give a brief history of the collection of admission data (outlined in Box 1) and discuss research studies that used this information.