Purpose: The tumor mRNA expression levels of mammaglobin, a novel breast-specific and breast cancer-associated marker, were correlated with disease outcome in 280 patients with primary breast cancer. Patients and Methods: Mammaglobin expression levels were assessed by quantitative reverse transcriptase polymerase chain reaction in frozen tumor tissue from breast cancer patients with a median age of 60 years (range, 30 to 88 years) and a median follow-up of 85 months (range, 2 to 169 months). Results: High expression levels were associated with low-grade tumors (P = .018), with positive estrogen and progesterone receptor status (P < .001), and postmenopausal status (P = .010). In the analysis of all patients, low tumor mammaglobin expression levels predicted an early relapse both in Cox univariate (hazard ratio [HR], 0.52; 95% CI, 0.34 to 0.79; P = .002) and multivariate regression analyses corrected for the traditional prognostic factors (HR, 0.55; 95% CI, 0.35 to 0.88; P = .012). The association of mammaglobin expression with the rate of relapse was particularly favorable in patients who received adjuvant tamoxifen treatment (HR, 0.35; 95% CI, 0.17 to 0.71; P = .004). Conclusion: These results demonstrate that the assessment of the tumor mRNA expression level of the breast-specific protein mammaglobin could be useful to stratify patients for individual adjuvant treatment strategies.