Using the Danish Civil Registration System, the authors identified all women under 50 years of age who were treated for cancer of the breast in Denmark between 1982 and 1998. The aim was to determine if the survival of young women with breast cancer was affected by the treatment modality used, either total mastectomy plus axillary dissection or lumpectomy plus axillary dissection followed by radiotherapy. Patients were considered high-risk if they had positive lymph nodes, tumor size >5 cm, or histologic grade II or III disease. Conservative therapy was available only in experimental trials before 1989. A total of 7165 patients had undergone radical mastectomy and 2120 had lumpectomy. Compared with women over 35 years of age, patients less that 35 years of age were more likely to have tumor size <2 cm and to have positive lymph nodes (P = 0.007 and P = 0.002, respectively). Lumpectomy was more likely to be used in women with smaller tumor size (<2 cm) and in those whose lymph nodes were negative for disease (P <0.001 for both). Using women who had radical hysterectomy as a reference group, a multivariate analysis of possible risk factors for survival was performed for women who were treated with conservative therapy. Surgical treatment, tumor size, axillary lymph node status, histologic grade, year of treatment, and protocol allocation were included in the analysis. There was no variable, including breast-conserving therapy, that carried a predictive value for breast cancer survival. In this study, a significantly lower risk for death from breast cancer was seen in women 45 to 49 years of age who underwent lumpectomy and radiation therapy compared with radical mastectomy (relative risk, 0.66; 95% confidence interval, 0.50–0.88), even when the analysis was restricted to patients with smaller tumor size or to those who were randomly assigned to conservative therapy. Younger women <35 years of age who had conservative therapy had a significantly greater incidence of local recurrence than those in the 45- to 49-year age range (15.4% vs. 3.0%). A total of 4210 patients were considered low-risk and, therefore, did not receive adjuvant chemotherapy. Among these women, those who underwent breast-conserving therapy were less likely to survive, but the trend did not reach significance.