Not all sites performing mammography produce optimal images at acceptably low doses. To address this situation, a voluntary accreditation program was instituted through the American College of Radiology. Each site applying for accreditation completes a detailed questionnaire about facilities and equipment, personnel, and quality control. Each unit at each site is evaluated by a medical physicist for clinical and phantom image quality and for average glandular dose. Although a site is accredited for 3 years, the performance of each unit is audited annually. Of 11,162 units for which accreditation was sought, only 5,856 (68%) passed at the first attempt, with another 18% passing after reapplication. The major reason for failure was inadequate clinical images, followed by inadequate phantom images, both inadequate clinical and phantom images, and inadequate quality control records. In a performance audit of 20 new units, an average of 5.8 problems were encountered per unit, the majority of which adversely affected image quality. These results support the requirement of equipment performance audits and raise the question of whether similar problems exist in other areas of medical imaging.