PURPOSE To retrospectively measure repeat rates for high-cost imaging studies, determining their causes and trends, and the impact of radiologist recommendations for a repeat examination on imaging volume. MATERIALS AND METHODS This HIPAA-compliant study had institutional review board approval, with waiver of informed consent. Repeat examination was defined as a same-modality examination performed in the same patient within 0 days to 7 months of a first examination. From a database of all radiology examinations (>2.9 million) at one institution from May 1996 to June 2003, a computerized search identified head, spine, chest, and abdominal computed tomographic (CT), brain and spine magnetic resonance (MR) imaging, pelvic ultrasonography (US), and nuclear cardiology examinations with a prior examination of the same type within 7 months. Examination pairs were subdivided into studies repeated at less than 2 weeks, between 2 weeks and 2 months, or between 2 and 7 months. Automated classification of radiology reports revealed whether a repeat examination from June 2002 to June 2003 had been preceded by a radiologist recommendation on the prior report. Trends over time were analyzed with linear regression, and 95% confidence intervals were calculated. RESULTS Between July 2002 and June 2003, 31 111 of 100 335 examinations (31%) were repeat examinations. Body CT (9057 of 20 177 [45%] chest and 8319 of 22 438 [37%] abdomen) and brain imaging (6823 of 18 378 [37%] CT and 3427 of 11 455 [30%] MR imaging) represented the highest repeat categories. Among five high-cost, high-volume imaging examinations, 6426 of 85 014 (8%) followed a report with a radiologist recommendation. Most common indications for examination repetition were neurologic surveillance within 2 weeks and cancer follow-up at 2-7 months. From 1997 to mid-2003, MR imaging and CT repeat rates increased (0.71% per year [P < .01] and 1.87% per year [P < .01], respectively). CONCLUSION Repeat examinations account for nearly one-third of high-cost radiology examinations and represent an increasing proportion of such examinations. Most repeat examinations are initiated clinically without a recommendation by a radiologist.