Breast carcinoma arose in or in conjunction with microglandular adenosis (MGA) in 14 of 60 (23%) patients with MGA listed in the authors' files. This article describes the clinicopathologic and immunohistochemical features and prognosis of these carcinomas. The median patient age was 47 years (range, 26-68 years). All patients had a mass. Six (43%) had a family history of breast carcinoma. Lymph node metastases were found in 3 of 11 axillary dissections. Ten patients treated by mastectomy were recurrence-free, with a median follow-up of 57 months (range, 3-108 months). Two of three patients treated by excisional surgery were recurrence-free 12 and 105 months later. The third woman had bone metastases at 51 months and was alive 98 months after treatment. Carcinoma arose in the MGA in 13 patients. In these patients, in situ carcinoma was found in expanded MGA glands composed of cells with vesicular poorly differentiated nuclei. One patient with benign MGA had carcinoma develop in the opposite breast that was not associated with MGA. When it arose in MGA, basement membranes were present in benign MGA and in situ carcinoma but tended to be disrupted in invasive foci that appeared to be formed by coalescent MGA glands. Strong immunoreactivity for cytokeratin, S-100, and cathepsin D was detected in carcinomas. Two carcinomas had nuclear progesterone receptors, and one of these had estrogen receptors. One carcinoma had positive findings for HER-2neu, and four had immunoreactivity for p53 protein. The following conclusions were drawn from these observations: (1) carcinomas arising in MGA have a distinctive histopathologic pattern; (2) the carcinomas are composed of epithelial cells (cytokeratin positive, actin negative) that are strongly immunoreactive for S-100 protein and cathepsin D; and (3) with a median follow-up of nearly 5 years, patients with these carcinomas had a relatively favorable prognosis, despite histopathologic and immunohistochemical features usually associated with a poor prognosis.