Timely cancer diagnosis remains an important area for improvement. Using a national breast cancer registry, researchers reread negative screening mammograms of women diagnosed with breast cancer in the two years following the screening. Screening mammograms were then rated as true negative (i.e., no cancer could be detected) or missed (i.e., signs of cancer were visible but missed). Among women with screen-detected or interval cancer, most initial screening mammograms did not show visible signs of cancer.