This paper describes the evolution of hospital information systems (HIS) in the United States of America from 1950 to the present and defines HIS within the context of a medical information system. After a concise review of HIS administrative functions, the paper focuses on HIS clinical functions, including the following: computer-based patient records; data entry and retrieval; text processing; data and system protection; networks; clinical subsystems (nursing and medical subspecialties); clinical decision support and quality assurance; and research databases. The paper surveys early examples of HIS and makes projections for HIS in the 1990s.