We analyzed the characteristics of patients with breast cancer detected by mass screening over the last 15 years in Gunma Prefecture. The patients were divided into three groups : A, patients seen from 1980 to 1984 ; B, patients seen from 1985 to 1989 ; C, patients seen from 1990 to 1994. The control group comprised patients with breast cancer patients detected in outpatient clinics (group D). As the rate of repeated participants in the screening and monthly self-examined patients had increased, the average tumor size in groups B and C was smaller than that in groups A and D. Clinical stage and survival rate in groups B and C were better than those in groups A and D. Although average tumor size, clinical stage and survival rate of all the patients with cancers detected by screening were improved, the average size of tumors detected by initial screening became larger, since about half of these patients had been aware of a lump in their breast and had first participated in the screening. It is necessary to inform women to consult a physician immediately when they become aware of a lump on their breast.