BACKGROUND The multidrug resistance protein (MRP1) is expressed in human breast carcinomas but its clinical significance remains unclear. The aim of the present study was to determine the clinical significance of MRP1 in breast cancer patients. METHODS MRP1 expression of primary carcinomas from 100 breast cancer patients was immunohistochemically determined by means of the monoclonal antibodies QCRL-1/QCRL-3. RESULTS MRP1 was negative in 20 (20%) and positive in 80 (80%) breast carcinomas. MRP1 expression was more frequent in both estrogen receptor-negative carcinomas and progesterone receptor-negative carcinomas (p = 0.1 in both cases), but was independent of tumor size and lymph node involvement. Patients with MRP1-negative carcinomas had prolongations of overall survival (p = 0.01 for death due to any cause, p = 0.04 for breast cancer-related death) and disease-free survival (p = 0.07) as compared to those with MRP1-positive carcinomas. Also in subsets of patients (negative lymph nodes; positive lymph nodes; positive estrogen receptor; T1/T2 tumors), overall survival was longer for patients with MRP1-negative carcinomas. In univariate Cox regression analyses, MRP1 positivity was associated with relative risks of 4.9 (95% CI 1.2-20.6; p = 0.03) for death due to any cause, 6.4 (95% CI 0.9-48.0; p = 0.07) for breast cancer-related death and 3.5 (95% CI 0.8-14.9; p = 0.09) for relapse. In multivariate Cox regression analyses, MRP1 positivity had relative risks of 5.1 (95% CI 1.2-21.7; p = 0.03) for death due to any cause, 6.5 (95% CI 0.8-50.1; p = 0.07) for breast cancer-related death and 3.4 (95% CI 0.8-15.1; p = 0.1) for relapse. CONCLUSIONS Our results suggest that MRP1 might be an important factor in breast cancer indicating excellent prognosis for patients with MRP1-negative carcinomas.