PURPOSE: The use of mammographic screening has led to the early detection of breast cancers as well as the increasing incidence of ductal carcinoma in situ (DCIS) and DCIS with microinvasion (MI). The biologic behaviors and management of DCIS and DCIS with MI remain uncertain and controversial. We designed this study to investigate the differences in clinical behavior and association with pathological parameter of both DCIS and DCIS with MI. METHODS: DCIS with MI was defined as DCIS with and invasive area of 1 mm or less in greatest dimension. We analyzed and compared the clinico-pathological features and treatment outcomes of 155 DCIS patients and 73 DCIS with MI patients. Chi-square test, student t-test and Kaplan-Meier method using SPSS 9.0 for MS-windows were used to verify the statistical significance. RESULTS: Both DCIS with MI and DCIS were most prevalent in women in the fifth decade, and the mean ages of the two groups were 45.0 and 46.8 years old, respectively. The primary tumors of DCIS with MI were more palpable (72.6% vs. 56.8%, P=0.032) upon physical examination and larger (3.1+/-0.21 cm vs. 2.6+/-0.12 cm, P=0.037) than those of the DCIS group. The rate of axillary lymph node metastasis was higher in the DCIS with MI group (8.3% vs. 0.7%, P=0.003). The DCIS with MI group was more commonly associated with high nuclear grade (50% vs. 28%, P=0.028). The DCIS with MI group was also linked with comedo type, although not to a statistically significant degree (67.6% vs. 52.6%, P=0.095). In terms of hormone receptor, there was no significant difference between the groups. There were three systemic metastases in DCIS patients and two DCIS with MI patients (P>0.05). There were no local-regional recurrences in either groups. The 8-year disease-free survival rates of the DCIS and DCIS with MI groups were 98.1% and 95.8% respectively (P>0.05). CONCLUSION: DCIS with MI has several clinical-pathological characterisitcs: more palpable on physical examination, larger in size, higher incidence of lesions with comedo necrosis and high nuclear grade. Examination of the axillary lymph node with less invasive techniques may be necessary in cases with suspicious invasion. Since DCIS with MI is thought to be a transitional disease entity between DCIS and invasive ductal carcinoma and has a metastatic potential, a careful histologic evaluation is necessary for the diagnosis of DCIS.