Background: The Surgery Versus Active Monitoring for Low-Risk DCIS (LORIS) trial is studying the safety of monitoring core-biopsy diagnosed low-risk ductal carcinoma in situ (DCIS) without excision. We sought to determine the incidence and characteristics of synchronous invasive carcinoma found in LORIS-eligible women who underwent excision, as this knowledge is essential in assessing the safety of observation alone. Methods: Women meeting LORIS eligibility criteria (age ≥46 years, screen-detected calcifications, non-high-grade DCIS diagnosed by core biopsy, absence of nipple discharge, or strong family history of breast cancer) who underwent surgical excision from 2009 to 2012 were identified. Histologic findings of excision specimens were reviewed. Results: Overall, 296 LORIS-eligible cases were identified; 58 (20 %) had invasive carcinoma on final pathology (90 % invasive ductal, 78 % >1 mm in size, 21 % high grade, 3 % triple negative, 9 % HER2 amplified). Of these, 18 (31 %) were pT1b or larger and 3 (5 %) were pN1. Among eligible upgraded cases, 90 % received radiation, 89 % received endocrine therapy, and 18 % were recommended chemotherapy. Women upgraded to invasive carcinoma were more likely to have intermediate-grade DCIS on core biopsy and to have undergone mastectomy. Conclusions: Among LORIS-eligible women, 20 % had invasive carcinoma at surgical excision that was heterogeneous in grade, size, and receptor status. Information gained from surgical excision influenced receipt of adjuvant radiation and endocrine therapy in most patients, and indicated benefit from chemotherapy in 18 % of patients. Surgical excision is warranted until additional risk stratification is available to identify a cohort of DCIS patients at lower risk for clinically significant synchronous invasive carcinoma.