Purpose: Many patients with unilateral breast cancer choose contralateral prophylactic mastectomy to prevent cancer in the opposite breast. The purpose of our study was to determine the use and trends of contralateral prophylactic mastectomy in the United States. Patients and Methods: We used the Surveillance, Epidemiology and End Results database to review the treatment of patients with unilateral breast cancer diagnosed from 1998 through 2003. We determined the rate of contralateral prophylactic mastectomy as a proportion of all surgically treated patients and as a proportion of all mastectomies. Results: We identified 152,755 patients with stage I, II, or III breast cancer; 4,969 patients chose contralateral prophylactic mastectomy. The rate was 3.3% for all surgically treated patients; 7.7%, for patients undergoing mastectomy. The overall rate significantly increased from 1.8% in 1998 to 4.5% in 2003. Likewise, the contralateral prophylactic mastectomy rate for patients undergoing mastectomy significantly increased from 4.2% in 1998 to 11.0% in 2003. These increased rates applied to all cancer stages and continued to the end of our study period. Young patient age, non-Hispanic white race, lobular histology, and previous cancer diagnosis were associated with significantly higher rates. Large tumor size was associated with a higher overall rate, but with a lower rate for patients undergoing mastectomy. Conclusion: The use of contralateral prophylactic mastectomy in the United States more than doubled within the recent 6-year period of our study. Prospective studies are needed to understand the decision-making processes that have led to more aggressive breast cancer surgery.