The objective of this study is to analyse detection rates and stage of diagnosis of interval cancers in the mass screening mammography programme of Isère (France), launched in 1990. Interval cancers are defined as breast cancers diagnosed within 30 months after a negative screening assessment, for women attending the programme between November 1990 and December 1994. Stages of diagnosis of these cancers are compared with those of screened cancers and to those of cancers diagnosed outside the programme. The rates of invasive interval cancers are 17.7% of the expected incidence rate during the first year, 60.0% during the second year and 58.8% after the second year. Sensitivity of the programme (one test every 30 months) is 74%; sensitivity at one year is 82%. Results are better for women aged 60-69 years than for younger women (50-59 years). Diagnosis is made at an early stage with 8% of in situ cases, and with 40% of very small tumours (sizes < or = 10 mm). Those stages are very close to the ones for screened cases. Interval cancer rates are low during the first year. Higher rates for the second year and early stages of diagnosis could be explained by self-referred screening practice in our area.