Guidelines for screening mammography were first suggested by the American Cancer Society in 1977. These have been reviewed periodically, and changes have been made based on the evidence available at the time of review. Two major breast screening studies conducted in the United States have had strong influences on the guidelines. The Health Insurance Plan of Greater New York demonstrated a constant decrease in mortality for women over the age of 50, a finding that was apparent 5 years after the study's completion. At that time, no significant change could be demonstrated in the 40-49-year age group; however, 10 years after the study's conclusion, the decrease for the younger age group had increased to 23.5% and has remained essentially constant. Despite the small number of patients involved, the American Cancer Society considers this decrease in women aged 40-49 years significant. The Breast Cancer Detection Demonstration Project, sponsored by the American Cancer Society and the National Cancer Institute, involved 283,222 women and was conducted under guidelines developed by the Consensus Development Meeting in 1977. The 14-year follow-up for this study continues to show a survival rate in the 80th percentile for women aged 40-49 years. In 1987, 12 organizations examined all existing guidelines in an attempt to develop uniform recommendations that would eliminate some of the confusion surrounding mammography screening. In 1991, following a 2 1/2-day conference sponsored by the American Cancer Society, it was concluded that current evidence did not indicate that a change in the screening guidelines was necessary, with the exception of the omission of the previously recommended baseline study at the age of 35. Future reviews will be conducted as pertinent information becomes available.