The aim of the present study was to monitor the occurrence of interval (breast) cancers (IC) as an interim outcome measure of quality assurance in the mammographic screening programme of the District of Florence. IC were identified by linking breast screening data with registrations of breast cancers from the Tuscany Tumour Registry. The main outcome measures were the numbers, rates, and the proportionate incidence of IC. Overall, 90 invasive IC were observed among women who attended the breast screening programme in the District of Florence from 1 January 1985 to 31 December 1993, respectively, 23, 50, on 17 cases, at 1, 2, and 3 years after the last negative screening test. Expressed as a proportion of underlying incidence, the figures are 16, 44, and 46%. As to risk factors for IC, mammographic density was the most important one in the present survey, whereas no effect of age emerges from the present study.