To investigate the characteristics and prognosis of interval breast cancer (IBC) appearing during mass screening examinations involving questions, inspection and palpation, our mass screening system in Osaka Prefecture was evaluated. In Osaka Prefecture, cancer registration was established in 1962, and therefore we used the data from this register to find interval breast cancers among the mass screening group. <BR>From April 1981 to March 1988, 15, 385 women were screened. Fifteen IBCs (0.1%) and 37 screening-detected breast cancers (SBCs) (0.24%) were found during this period. The only complaint about self-detected breast mass was effective for the separation of IBC from SBC. There was no significant difference between IBC and SBC in clinical findings, mammography data, echography findings, clinical stage or pathological findings. <BR>Overall survival was almost the same in the IBC and SBC groups, but IBCs detected in the first year after the last screening had a better prognosis than those detected in the second year. These results suggest that annual mass screening examination involving question, inspection and palpation should be carried out.