Low-grade adenosquamous carcinoma of the breast is an uncommon neoplasm of uncertain pathogenesis, clinical behavior, and malignant potential. This report describes the clinical and pathologic features of 32 cases of low-grade adenosquamous carcinoma. All patients presented with palpable tumors ranging from 0.6 to 8.6 cm (mean, 2.8 cm). Origin from an intraductal papillary tumor was found in 12 cases, including three with adenomyoepitheliomatous features. Electron microscopy disclosed glandular and squamous differentiation; the squamous cells often lined ducts that were structurally analogous to the acrosyringium of the eccrine sweat gland. Treatment consisted of mastectomy (13 patients) or excisional biopsy (19 patients). A single lymph node in one patient with a 3.5-cm primary carcinoma harbored metastatic adenocarcinoma. Axillary dissection revealed no metastases in 11 other patients. Another patient with an 8.0-cm breast tumor had metastatic adenosquamous carcinoma in the lung at initial diagnosis. After follow-up of 12 to 124 months, 20 of 25 patients had no recurrence. Five women treated by excisional biopsy had local recurrences in the breast. In one patient, the local recurrence was ultimately fatal due to invasion of the hemithorax. Estrogen and progesterone receptor studies were negative in 13 of 15 cases studied by biochemical analysis. The two tumors that were hormone-receptor positive were histologically associated with a papilloma and an adenomyoepithelioma, respectively. In the latter case, immunohistochemical studies showed the carcinoma to be hormone-receptor negative. Hormone receptor activity was limited to the adenomyoepitheliomatous component. This study confirms the largely indolent, but locally aggressive, clinical course of low-grade adenosquamous carcinoma of the breast. Although complete limited excision of small lesions may be curative, tumors greater than 3.0 cm may require more aggressive therapy.