The age-specific sensitivity of a screening program was investigated using a population-based cancer registry as a source of false-negative cancer cases. A population-based screening program for breast cancer was run using either clinical breast examinations (CBE) alone or mammography combined with CBE in the Miyagi Prefecture from 1997 to 2002. Interval cancers were newly identified by linking the screening records to the population-based cancer registry to estimate the number of false-negative cases of screening program. Among 112 071 women screened by mammography combined with CBE, the number of detected cancers, false-negative cases and the sensitivity were 289, 22 and 92.9%, respectively, based on the reports from participating municipalities. The number of newly found false-negative cases and corrected sensitivity when using the registry were 34 and 83.8%, respectively. In detected cancers, the sensitivity of screening by mammography combined with CBE in women ranging from 40 to 49 years of age based on a population-based cancer registry was much lower than that in women 50-59 and 60-69 years of age (40-49: 18, 71.4%, 50-59: 19, 85.8%, 60-69: 19, 87.2%). These data suggest that the accurate outcome of an evaluation of breast cancer screening must include the use of a population-based cancer registry for detecting false-negative cases. Screening by mammography combined with CBE may therefore not be sufficiently sensitive for women ranging from 40 to 49 years of age.