BACKGROUND: In ductal carcinoma in situ of the breast (DCIS), histologic diagnosis obtained before the definitive treatment is related to the risk of underestimation if the presence of invasive cancer is found postoperatively. These patients need a second operation to assess the nodal status. We evaluated the upstaging rate in patients with mass-forming DCIS. METHODS: Sixty-three women with pure DCIS presenting as sonographic mass lesion underwent vacuum-assisted or core-needle biopsy and subsequent surgery. Rates of postoperative upstaging to invasive cancer were calculated and compared with clinical character and size of DCIS. RESULTS: Median age of patients (range) was 63 years (27-88) while median diameter of DCIS was 11 mm (6-60). Fifty-six percent of DCIS were upstaged. Patient age did not differ significantly between groups with and without final invasion (median, mean, SD): 63, 61.4, 12.5 vs 62, 61.2, 10.6 years, respectively (P=0.659). The difference of DCIS size between these groups was statistically important (median, mean, SD): 13, 17.3, 11.4 vs 9.5, 9.8, 3.2 mm, respectively (P=0.0003). Mass size and palpability were significant risk factors (P<0.001 and P<0.01, respectively). Rate of underestimation for mass with diameter≤10 mm, 10-20 mm and >20 mm was 37, 64 and 91%, respectively. CONCLUSIONS: DCIS diagnosed on minimal-invasive biopsy of even small sonographic mass is of high risk for the upstaging to invasive cancer after final surgical excision. In these patients, subsequent intervention is needed for nodal status assessment. They are good candidates for the sentinel node biopsy during the breast operation to avoid multi-step surgery.