Syringomatous squamous tumors of the breast. Four cases are reported of syringomatous squamous tumors of the breast occurring in women aged 37 to 70 years. The lesions were characterized histologically by relatively well-circumscribed tumor-like nodules composed of a proliferation of teardrop or comma-shaped islands of squamous epithelium. The squamous epithelial islands contained central lumens lined by eosinophilic cuticles and were surrounded by a densely cellular fibrous matrix, thus closely resembling the growth pattern of dermal eccrine syringomas. The lesions appeared to arise de novo from breast parenchyma without evidence of transitions with the surrounding normal or hyperplastic mammary epithelium and were not associated with the overlying skin or nipple epidermis. In all cases, the surrounding breast tissue showed fibrocystic and benign proliferative changes, and in one case, the lesion was found in the vicinity of a large cyst surrounded by microcalcifications. All patients were treated by local surgical excision and have shown no evidence of recurrence over a follow-up period of 1 to 6 years. The histologic differential diagnosis and the possible pathogenesis of these lesions are discussed. 