Ductal carcinoma in situ of the breast is a heterogeneous group of lesions with diverse malignant potential. It is the most rapidly growing subgroup in the breast cancer family; it is projected that more than 39,000 new cases will be diagnosed in the United States during 1999. Most new cases are nonpalpable and are discovered mammographically. Treatment is controversial and ranges from excision only, to excision with radiation therapy, to mastectomy. Genetic changes routinely precede morphologic evidence of malignant transformation. Medicine must learn how to recognize these genetic changes, exploit them, and in the future, prevent them.