In response to the Omnibus Reconciliation Act of 1987 mandate for the development of a national resident assessment system for nursing facilities, a consortium of professionals developed the first major component of this system, the Minimum Data Set (MDS) for Resident Assessment and Care Screening. A two-state field trial tested the reliability of individual assessment items, the overall performance of the instrument, and the time involved in its application. The trial demonstrated reasonable reliability for 55% of the items and pinpointed redundancy of items and initial design of scales. On the basis of these analyses and clinical input, 40% of the original items were kept, 20% dropped, and 40% altered. The MDS provides a structure and language in which to understand long-term care, design care plans, evaluate quality, and describe the nursing facility population for planning and policy efforts.