Breast cancer is the most commonly diagnosed cancer and the second leading cause of cancer-related death (after lung and bronchial cancer) among women in the United States. In 2002, at least 182,125 women in the United States had a diagnosis of invasive breast cancer, and 41,514 died from the disease. Screening mammography can reduce mortality from breast cancer by approximately 20%-35% in women aged 50-69 years and approximately 20% in women aged 40-49 years. Organizations including the American Medical Association, American College of Obstetricians and Gynecologists, and American Cancer Society support mammography screening beginning at age 40 years, although these groups vary in their recommendations regarding intervals for rescreening. The U.S. Preventive Services Task Force, an independent panel of private-sector experts in prevention and primary care convened by the Department of Health and Human Services, recommends that women aged < or = 40 years be screened for breast cancer with a mammogram every 1-2 years. Although mammogram use increased substantially during the 1990s, results from a recent cohort study of health maintenance organization members revealed declining screening rates during 1999-2002. This report describes Behavioral Risk Factor Surveillance System (BRFSS) findings that indicate a similar decreasing trend in self-reported use of mammograms among women aged < or = 40 years during 2000-2005. Continued declines in mammography use might result in increased breast cancer mortality.