Surgery of the primary tumor usually is not advised for patients with metastatic breast cancer at diagnosis because the disease is considered incurable. In this population-based study, we evaluate the impact of local surgery on survival of patients with metastatic breast cancer at diagnosis. We included all 300 metastatic breast cancer patients recorded at the Geneva Cancer Registry between 1977 and 1996. We compared mortality risks from breast cancer between patients who had surgery of the primary breast tumor to those who had not and adjusted these risks for other prognostic factors. Women who had complete excision of the primary breast tumor with negative surgical margins had a 40% reduced risk of death as a result of breast cancer (multiadjusted hazard ratio HR, 0.6; 95% CI, 0.4 to 1.0) compared with women who did not have surgery (P = .049). This mortality reduction was not significantly different among patients with different sites of metastasis, but in the stratified analysis the effect was particularly evident for women with bone metastasis only (HR, 0.2; 95% CI, 0.1 to 0.4; P = .001). Survival of women who had surgery with positive surgical margins was not different from that of women who did not have surgery. Complete surgical excision of the primary tumor improves survival of patients with metastatic breast cancer at diagnosis, particularly among women with only bone metastases.