Telemedicine is one proposed solution to problems of accessibility, quality, and costs of medical care. Although telemedicine applications have proliferated in recent years, their diffusion has remained low in terms of the volume of consultations. In this study, Attewell’s (1992) theory of knowledge barriers is extended to explain why diffusion of telemedicine remains low. In case studies of telemedicine programs in three world- renowned medical centers in Boston, Massachusetts, we find that, in addition to technical knowledge barriers, as suggested by Attewell, there are economic, organizational, and behavioral knowledge barriers that inhibit the diffusion of telemedicine. The lowering of these barriers entails intensive learning efforts by proponents of applications within adopter organizations. They need to develop technically feasible, medically valid, reimbursable, and institutionally supported applications in order to justify the value of telemedicine and engender frequent and consistent use by physicians.