The Online Medical Record (OMR) is a full-featured shared electronic patient record in use since 1989 at Beth Israel Deaconess Medical Center in Boston. The first practice to use the OMR was a primary care practice. We observed the pattern of voluntary adoption of the OMR and the referral patterns from primary care to specialists. Adoption of the OMR among specialists has accelerated in recent years, in many cases mirroring the referral patterns from primary care to specialists. We hypothesize that referral of patients from primary care providers to specialists exposes these specialists to the benefits the electronic patient record and may promote the use of this technology. We conclude that these referral patterns provide a vector for the dissemination of electronic patient records. The important lesson is that EPR implementation in a health care network should begin with primary care to ensure the most efficient diffusion of this technology throughout the enterprise.