A mammographic breast cancer screening programme has been ongoing in the Florence District (Italy) since 1970 and a favourable impact of screening on breast cancer mortality of women aged 50-70 has been shown by means of a case-control study. Two hundred and eleven screen- and 116 interval-detected cancers in the period 1975-1986 have been identified, and detection rates calculated, for first and repeated screening test (2nd to 7th). Overall, 22,980 subjects were screened and 44,988 repeated tests performed. The observed number of interval cancers has been compared with the expected incident cancers and their ratio (O/E) studied at different time intervals since last test. The O/E ratio at the third year since the last test was 0.98 for the age-group 40-49 0.50 (95% CI: 0.23-0.95) and 0.39 (95% CI: 0.26-0.94) for the 50-59 and 60-69 groups, respectively. The prevalence/incidence ratio (P/I) was then calculated as an early indicator of efficacy. For the 40-49 age-group the P/I ratio at first test was 1.09, suggesting poor anticipation of diagnosis. In contrast, for women 50-59 and 60-69 results suggest quite a good diagnosis anticipation (P/I: 3.14; 4.82), confirming the result of the previous case-control study on mortality reduction. The proportion of advanced carcinomas (stage II or worse) and 5-year survival have been analysed and discussed. The study confirms the opportunity of using early indicators of screening efficacy for monitoring of screening services.