A review of 6109 mammograms representing 4332 patients seen over 4 years is presented. The American Cancer Society guidelines for mammography screening were followed. Forty-five percent of all tumors were nonpalpable; in 12% of these the lymph nodes were positive for cancer. In the group with palpable breast cancers, 62% had positive lymph nodes. Screening mammography results in an increased number of localization biopsies. Biopsy rates were compared with several series. Costs for mammography screening programs were reviewed and related to the cost of each breast cancer detected. In our series the cost per cancer detected was $13,000. This study provides evidence to justify the American Cancer Society guidelines.