Breast cancer is a major cause of death among black women. At Cook County Hospital in Chicago, a hospital serving a predominantly black population, only 31% of breast cancers were localized at the time of diagnosis in the years 1980-1983. A survey of patient records in the largest primary care clinic in the hospital revealed that only 2% of eligible patients had received mammograms in 1982. A nurse-run breast cancer detection program was begun in mid-1983. Since that time the proportion of women who have received periodic breast examinations has increased from 26% to 46% (P less than 0.001) and periodic mammograms have increased from 2% to 41% (P less than 0.001). The proportion of women having received breast self-examination teaching increased from 10% to 58% (P less than 0.001). The proportion of localized breast cancer was significantly greater for those women whose cancer was diagnosed through the screening program (44 of 72, 61%) compared to other clinical areas (71 of 213, 33%) in the years 1984 through 1986 (P less than 0.001). There has been a significant increase in the percentage of localized breast cancer at Cook County Hospital comparing 1980-1983 statistics (31%), before the Breast Cancer Detection Program was fully established, with 1984-1986 statistics (40%) (P less than 0.02).