Melatonin inhibited the proliferation of hormone-independent LNCaP prostate cancer cells partly via MT1 receptor activation both in vitro and in nude mice xenograft model. In this study, the melatonin receptor expression in the prostate cancer tissue of a patient with bone metastases and the effect of melatonin on the biochemical progression of hormone-refractory prostate tumor which later developed in the same patient were reported. Saturation and competition 2-[125I]iodomelatonin binding assays were conducted on prostate tumor tissue obtained by transurethral resection of the prostate from the index patient. The receptor subtype identity of melatonin receptor expressed in the cancer tissue was determined by comparison of the rank order of inhibition constants (Ki) of various melatonergic ligands and the affinity of 4-phenyl-2-propionamidotetraline relative to melatonin in inhibiting 2-[125I]iodomelatonin binding to the tumor sample and to human cell lines stably transfected with MT1 or MT2 melatonin receptor subtype. MT1 receptor expression in the cancer tissue was also examined by immunohistochemistry. The surgically castrated patient later developed biochemical relapse of his disease. His serum total prostate-specific antigen (PSA) level was monitored before and during treatment with 5 mg/day oral melatonin at 20:00 hr. High-affinity (Kd = 103.7 pm) MT1 melatonin receptor subtype was expressed by the patient's prostate cancer. As indicated by his PSA levels, melatonin induced stabilization of his hormone-refractory disease for 6 wk. This report validates melatonin's oncostatic action on prostate cancer and the potential involvement of MT1 receptor subtype in the attendant antiproliferative signal transduction as suggested by recent preclinical laboratory findings in a human.