Purpose: The prognosis and factors that influence prognosis following salvage mastectomy in patients with recurrence in the treated breast after conservative surgery (CS) and radiation therapy (RT) were investigated. Materials and Methods: A total of 1,593 patients with stage I or II invasive breast cancer were treated following gross total excision of the tumor at the Joint Center for Radiation Therapy (JCRT) between 1968 and 1985. One hundred sixty-six of the 1,593 (10%) had subsequent recurrence in the breast. Of these, 123 had salvage mastectomy and constitute the study population. The recurrent tumor was predominantly invasive in 99 patients, noninvasive in 14, and focally invasive in 10. Following mastectomy, chemotherapy or hormonal therapy was administered to 29 patients. The median follow-up time was 39 months after salvage mastectomy. Results: The 5-year actuarial rate of further local or distant relapse for the entire group was 41 %. None of the 24 patients with focally invasive or noninvasive tumors had a subsequent relapse. In comparison, the 5-year actuarial rate of further relapse in the 99 patients with a predominantly invasive recurrence was 52% (P = .001). The method of detection of the recurrence, the age of the patient at initial diagnosis, the disease-free interval, and the location of the recurrence in the breast were not found to have a statistically significant association with the risk of further relapse. Conclusion: We conclude that the histology of the recurrent tumor is an important prognostic factor for the risk of further relapse. Patients with purely noninvasive or focally invasive tumors have an excellent prognosis following salvage mastectomy. In contrast, patients with predominantly invasive tumors are at substantial risk for further relapse.