It has been estimated that approximately 1 in 9 women (living to age 85 years) in the United States will develop breast cancer at some time in their lives; hence the interpretation of breast biopsies has become a large and important component of the surgical pathologist's practice. Indeed, there is great pressure on surgical pathologists to make accurate diagnoses of breast lesions because of their high incidence, the increased public awareness of breast disease, the greater use of screening mammography to detect early carcinomas, the development of multiple therapeutic options (which is often determined by tumor pathology), and the harsh medical-legal climate. The focus of this article will be to define the clinicopathologic features of two malignant breast lesions that are particularly prone to simulate benign lesions, and thus pose important diagnostic problems to surgical pathologists. The two lesions include spindle-cell carcinoma and low-grade duct carcinoma in situ.