In 1969 the Center for Social Diseases of Florence started a screening program for early breast cancer detection. The female population over 40 years of age of a group of outlying towns of the District was invited. From January 1969 till March 1977, 21,725 women have been examined in the program. Mammography was the diagnostic procedure of choice, followed by physical examination if necessary. Negative cases were controlled with biennial mammography. This paper summarizes and evaluates the results of this screening program. At first mammography, 67 cancers were detected, 37% of which were clinically unapparent, 62.3% staged T1A, and 52% N--. The average stage at diagnosis is certainly better than the average stage of cancers diagnosed in unscreened women, thus a better prognosis is expected. Actuarial survival rate of detected cancers was 94 +/- 3.7% at 5 years. False negative and false positive cases are reported. The possibility of hazards in the use of repeated mammography in mass screening is discussed. According to reported data the value of this screening program in terms of secondary prevention (early diagnosis) is confirmed.