Mammary Paget disease (MPD) is considered an intraepidermal manifestation of an underlying mammary carcinoma. In contrast to extramammary Paget disease, invasion of mammary Paget cells into the dermis (invMPD) has not been reported, except for 2 cases described in Rosen's textbook. Our study was designed to identify the presence of dermal invasion of mammary Paget cells and characterize the associated clinicopathologic features. Slides from 146 MPD patients were retrieved. Six cases of invMPD were identified. One case of invMPD was not associated with an underlying breast cancer, 1 was associated with invasive ductal carcinoma (IDC), 1 with ductal carcinoma in situ (DCIS) with microinvasion, and 3 with DCIS only. The underlying breast carcinomas was separate from the area of invMPD. The depth of invasion, measured from the dermal-epidermal junction to the focus of deepest invasion, ranged from 0.02 to 0.9 mm. The horizontal extent ranged from 0.01 to 4.0 mm. Lymph node with isolated tumor clusters was present in case 1, which had no underlying carcinoma but had the greatest extent of invasion, and in case 3, which had DCIS with microinvasion. One patient (case 1) died of unrelated causes 2 years later, and the remaining patients were alive without disease at last follow-up. In summary, we describe 6 cases of MPD with invasion of Paget cells into the dermis and provide histopathologic criteria for the diagnosis of this rare and underrecognized entity. Further studies are required to determine whether invasion in MPD has clinical significance.