The traditional goal of radiology has been to provide images of the highest technical quality that permit the most accurate diagnoses possible. From a more global perspective, diagnostic radiology is part of a larger system that has as its goal effective and efficient treatment of patients. A six-tiered hierarchical model of efficacy that includes this more global view is presented. This model is intended to improve understanding of the continuum that efficacy is and to conceptually relate efficacy to technology assessment and outcome research. Better understanding of efficacy/technology assessment and outcome research fostered by the model should encourage radiologists to participate actively in federally supported research in these areas. Those radiologists who do so will come to understand better how radiology can help patients and society as a whole. Use of the model's concepts should also aid radiologists in becoming more effective consultants to their clinical colleagues about optimal use of high-technology imaging from the perspectives of clinical efficacy, patient outcome, and societal efficacy.