The overuse of medical care is increasingly recognized as a patient safety issue. Overdiagnosis can result in unnecessary use of medical care, subjecting patients to greater risk of harm. For example, in the case of breast cancer, screening may detect lesions that are not clinically significant, leading to further testing and unnecessary procedures. This study examined the impact of mammography screening on a cohort of women in Denmark. Researchers found that screening was not associated with decreased incidence of advanced cancer but increased incidence of nonadvanced tumors and ductal carcinoma in situ; the rate of overdiagnosis was significant. An accompanying editorial discusses overdiagnosis in breast cancer.