This report describes the authors' experience in the design and implementation of two large scale picture archiving and communication systems (PACS) during the past 10 years. The first system, which is in daily clinical operation was developed at University of California, Los Angeles from 1983 to 1992. The second system, which continues evolving, has been in development at University of California, San Francisco (UCSF) since 1992. The report highlights the differences between the two systems and points out the gradual change in the PACS design concept during the past 10 years from a closed architecture to an open hospital-integrated system. Both systems focus on system reliability and data integrity, with 24-hour on-line service and no loss of images. The major difference between the two systems is that the UCSF PACS infrastructure design is a completely open architecture and the system implementation uses more advanced technologies in computer software, digital communication, system interface, and stable industry standards. Such a PACS can withstand future technology changes without rendering the system obsolete, an essential criterion in any PACS design.