BACKGROUND: Through the national survey of 1996, Korean Breast Cancer Society constructed the baseline data for breast cancer in Korea, and for each following year, through the same procedure, we want to understand changing patterns of breast cancer and establish appropriate plans for treatment and early diagnosis. The purposes of the present study were to determine changing patterns in the number of breast cancer patients age distribution, cancer stages, and methods of treatment surgeries compared to the data from 1996 and to determine differences in data among different countries by comparing these data with those from other countries. METHODS: In 1998, there were 41 medical schools in Korea, and 61 surgical training hospitals under Korean Surgical Society. Among these medical schools, data were collected from 36 medical schools except 5 schools that either did not have surgical department because they were recently established or where data collection was difficult, and among surgical training hospitals, from 41 hospitals except those that had less than 3 trainees or those that performed almost no breast cancer surgery. RESULTS: The total number of patients was 4,695 ; among these patients, the percentage of females was 99.7%, and that of males was 0.3%,1) The descending oder of the age distribution was 40's, 50's, 30's, 60's, 20's, 70's, 80's, and 10's with the youngest age of 16 years and oldest age of 89 years; and the median age was 47.0 years. 2)The operation method was MRM(70.1%), BCO(21.1%), simple mastectomy(2.8%), excision or biopsy(2.7%), RM(1.4%), and others(0.6%). 3)The AJCC stage was 0(6.4%), I(24.9%), II A(31.7%), II B(21.6%), III A(10.7%), III B(3.0%), and IV(1.7%). 4) The duration of symptoms was most frequent(32.1%) between one week to one month, and more than 3 years took up 2.7%. The order of symptoms was painless lump (72.1%), painful lump (9.5%), incidental finding of mammogram without any symptoms (8.7%), nipple discharge (5.1%), axiliary mass (4.0%), retraction of nipple or skin (3.3%), and breast pain (1.7%); the ratio of left and right breasts was 1.1:1, and the location was in the order of UOQ, UIQ, LOQ, Central, and LIQ. 5) The false-negative finding of mammography was 11.1%, and that of ultrasonography was 7.4%. The level of CEA was increased in 5.9% of the patients before surgery, and CaAg15-3 was increased in 7.2% 6) As earlier the cancer stage, less the destructive surgical method was used; and performing breast reconstruction was markedly different according to each hospital; and the percent of complications after surgery was 11.2% excluding of seroma. 7) The pathological diagnosis were IDC(89.2%), DCIS(5.6%), ILC(2.7%), other cancers such as lymphoma, sarcoma(2.2%), Paget's disease(0.3%), and LCIS(0.1%). Among the total number of patients, 43.0% showed lymph node metastasis, and 57.0% did not. 8) The positive rate of ER was 53.2%, and PgR was 44.8%. 9) Among the risk factors, the rate of family history of breast cancer was 5.0%, and those patients with the BMI score of more than 25 took up 32.5% of patients. Compared to th data from 1996, the present data showed 1) an increase in the number of patients, 2) relative increase of those women in their 20's, 3) decrease in mastectomy and increase in breast conserving surgery, 4) increase in the rate of early stage, 0 and 1, 5) among symptoms of breast cancer, increase in the rate of discovering breast cancer through regular mammography check-up, 6) increase in DCIS, 7) decrease those patients with lymph node metastasis, and 8) increase in those over-weight patients with the BMI score of more than 26. CONCLUSIONS: Several characteristics of breast cancer among Koreans are following the patterns of Western countries, and we need to understand the characteristics of breast cancer among Koreans through continuous investigations as the present study in the future.