Some of the earliest applications of computers in clinical medicine were in the clinical subspecialties, including cardiology, pulmonary, nephrology, gastroenterology, pediatrics, and the surgical sciences. But health professionals found these prototypes diffi cult to use; data entry devices were awkward and ineffi cient, and order entry functions were often not integrated. Each information system for a clinical subspecialty (ISCS) evolved differently, with its own specialized functional and technical requirements. In the 1960s mainframe computers were limited in their ability to meet all the processing requirements of all the ICSCs in a large hospital. By the 1970s each ISCS could have its own minicomputer-based system linked directly to the central mainframe. Health care professionals used terminals connected to the central computer to enter orders and to receive test results; the central computer transferred the orders to the appropriate ISCS subsystems and integrated the data coming back from the ISCSs into the patients' records stored in the mainframe computer. In the 1980s local area networks linked multiple lower-cost minicomputers; with distributed minicomputers and interactive visual display terminals, clinicians could begin to benefi t from the ISCSs in direct patient care and each ISCS could develop its own system to meet its own requirements. In the 1990s distributed information systems allowed physicians to enter orders and retrieve test results using clinical workstations connected to client-server minicomputers in the local area network that linked the entire hospital, and patient data from all of the distributed ISCS databases were integrated in a computer-based patient record.