Mammography studies, from 1963 through 1972, on 5,918 women over age 30 years with 327 breast cancer on initial studies, were prospectively categorized on a scale of 1 to 4 of increasing amounts of fibroglandular tissue. Approximately 60% of the cancers occurred in classes 1 and 2 breasts, about one-third of the patients, while 40% of the cancer were in the remaining two-thirds, comprising classes 3 and 4. There were 54 cancers that developed in breast that previously were free of symptoms, clinical signs, and x-ray abnormality. Up to 36 months one cancer was found in class 1, while 26 cancers were detected in class 4 breasts; two cancers developed in class 2 and seven in class 3. Cancers developing 38 to 88 months after normal examination had an incidence of 0.23% in combined classes 1 and 2 and an incidence of 0.21% in classes 3 and 4. Dense fibroglandular tissue delays detection of breast cancer by mammography. Apparent increase in cancer risk in such breasts is due to this delay. More than a 3-year follow-up is required to assess the life history of breast cancer by mammography.