Background: DCIS with microinvasion (DCISM) is a lesion for which prognosis may be intermediate between that of DCIS and invasive breast cancer, but for which the role of axillary lymph node staging remains controversial. Here we report clinical characteristics and outcome in 112 patients with DCISM, with a particular focus on the role of sentinel lymph node (SLN) biopsy. Methods: From our prospective database we retrospectively identified 112 patients with a diagnosis of DCISM who had undergone SLN biopsy between 1996 and 2004 at Memorial Sloan-Kettering Cancer Center. Median follow up was 6 years. Results: We found positive SLN in 12% (14/112) of all patients, macrometastases in 2.7% (3/112) and micrometastases in 10% (11/112). We performed axillary dissection (ALND) in all patients with macrometastases (3/3), finding additional positive nodes in 66% (2/3), and in 27% of those with micrometastases (3/11), finding no additional positive nodes. Among patients with negative SLN (38% of whom received systemic therapy), there were 5 loco-regional recurrences (1 in the ipsilateral axilla, and 4 in the ipsilateral breast, all DCIS) and 4 contralateral second primary breast cancers. Among patients with positive SLN (86% of whom received systemic adjuvant therapy), there were no loco-regional or distant recurrences. Conclusions: Positive SLN were present in 12% of our patients with DCISM, none of whom experienced recurrence at 6 years’ follow up. SLN biopsy may be justified for DCISM, but is clearly most beneficial to identify a very small subset of DCISM patients (2.7%, with SLN macrometastases) who could benefit from systemic adjuvant therapy. Our data imply that between 125 and 250 SLN biopsy procedures would be required to avoid breast cancer mortality in 1 patient, and do not support the routine use of ALND for SLN-positive patients. We recommend a critical reappraisal of routine SLN biopsy for DCISM.