The differential diagnosis of low-nuclear grade intraductal epithelial proliferations of the breast includes atypical ductal hyperplasia (ADH) and ductal carcinoma in situ (DCIS). This distinction can be difficult on core needle biopsy (CNB) but can have significant clinical ramifications. We examined the clinical course of patients diagnosed on CNB with borderline ADH/DCIS lesions [marked ADH (MADH)] at our institution. A total of 74 patients were diagnosed with MADH on CNB and underwent an excisional biopsy (EB). The majority of these CNBs reviewed at outside hospitals had been classified as DCIS. Twenty patients (27%) had benign findings or lobular neoplasia in their EB, 18 (24%) had ADH, 33 (45%) had DCIS, and 3 (4%) had DCIS and invasive ductal carcinoma (IDC). Among the 38 patients who were not diagnosed with DCIS or IDC on EB, no patient underwent further surgery or radiation postoperatively. Thirty-seven of these 38 patients had no recurrences, whereas 1 patient developed a "recurrence" that on our review was likely residual localized MADH. The mean follow-up for these patients was 54 months. Of the 36 patients diagnosed with DCIS or IDC on EB, <20% required mastectomy. On review, MADH involving an intermediate-sized duct on CNB and the amount of residual lesion on imaging was significantly associated with DCIS or IDC on EB. Conversely, MADH involving columnar cell lesions and the presence of calcification on CNB were significantly associated with benign pathology on EB. In conclusion, our study provides preliminary data that justify a conservative approach to borderline ADH/DCIS lesions on CNB: that is, diagnose as MADH and treat by conservative excision.