Background: Ductal carcinoma in situ with microinvasion (DCISM) is a rare diagnosis with a good prognosis. Although nodal metastases are uncommon, sentinel lymph node biopsy (SLNB) remains standard care. Volume of disease in invasive breast cancer is associated with SLNB positivity, and, thus we hypothesized that in a large cohort of patients with DCISM, multiple foci of microinvasion might be associated with a higher risk of positive SLNB. Methods: Records from a prospective institutional database were reviewed to identify patients with DCISM who underwent SLNB between June 1997 and December 2010. Pathology reports were reviewed for number of microinvasive foci and categorized as 1 focus or ≥2 foci. Demographic, pathologic, treatment, and outcome data were obtained and analyzed. Results: Of 414 patients, 235 (57 %) had 1 focus of microinvasion and 179 (43 %) had ≥2 foci. SLNB macrometastases were found in 1.4 %, and micrometastases were found in 6.3 %; neither were significantly different between patients with 1 focus versus ≥2 foci (p = 1.0). Patients with positive SLNB or ≥2 foci of microinvasion were more likely to receive chemotherapy. At median 4.9 years (range 0–16.2 years) follow-up, 18 patients, all in the SLNB negative group, had recurred for an overall 5-year recurrence-free proportion of 95.9 %. Conclusions: Even with large numbers, there was no higher risk of nodal involvement with ≥2 foci of microinvasion compared with 1 focus. Number of microinvasive foci and results of SLNB appear to be used in decision making for systemic therapy. Prognosis is excellent.