This study aimed to evaluate a program for the prevention and early diagnosis of breast cancer in a mobile mammography unit that visited several cities in Rio Grande do Sul, the southernmost State in Brazil, where the test was not available through routine services. The authors compared the characteristics of these patients with a group of women referred for mammography in the city of Pelotas, where such equipment is widely available. The mobile unit examined 8,607 women residing in 33 municipalities. The study identified 37 cases of breast cancer, or 4.7 cases per 1,000 tests. In Pelotas, 1,312 patients underwent mammography, and breast cancer prevalence was 6.9 cases per 1,000 tests. There were no statistical differences between the two prevalence rates, even after adjusting for potential confounders. The proposed intervention demonstrates that the availability of tests is needed for prevention and early diagnosis of breast cancer, determining the identification of cases when diagnosis would have been delayed without the mobile unit.