Objective: Multidrug resistance protein 4 (MRP4) a transmembranary transport protein has shown to be expressed in prostate cancer cell lines and cancer cell specimens and turned out to be among the highest upregulated genes in an arraybased transcription analysis. Its expression is regulated in an androgen dependent manner, possibly modulated by forkhead box A (FoxA), an androgen receptor (AR) co-activator. Therefore, we investigated its expression in a large cohort of neoplastic and non-neoplastic prostate tissues (n = 441) and evaluated its prediction of PSA relapse free survival (RFS). Method: TMA (n=441) stained for MRP4, FoxA, AR, ER, PSA, Ki-67: radical prostatectomies (RPE) (n=332), castrate resistant prostate cancer (CRPC) (n=26), metastases (n=39) and non-neoplastic (n=187). PSA-RFS of 258 patients; mean 70 months. Results: MRP4 expression decreases with tumor progression into castrate resistant disease, correlates with PSA, AR and FoxA expression and inversely correlates with Gleason score. Moreover, a strong MRP4 expression is significantly associated with a longer PSA RFS in RPE patients. Normal tissues from the transitional zone show a weak MRP4 expression compared to the peripheral and central zones. Conclusion: MRP4 expression seems to predict PSA relapse free survival in prostate cancer patients. Since its expression is androgen dependent it decreases with tumor progression into CRPC.