To evaluate the clinical usefulness of serum soluble Fas (sFas) and sFas ligand as a prognostic factor and for monitoring the regression and progression of metastatic prostate cancer treated with endocrine therapy, sFas and sFas ligand were measured in sera collected from 30 patients with untreated metastatic prostate cancer. sFas levels were measured sequentially in 16 patients who had progressed to a state of elevated prostate-specific antigen (PSA) and 5 patients who were in regression following endocrine therapy. Serum sFas levels in patients with metastatic prostate cancer were found to be significantly higher than that of control patients with benign prostate hyperplasia. Patients with low levels of serum sFas had a higher cause-specific survival rate and a higher PSA progression-free rate compared with patients with high levels of serum sFas. In patients who suffered PSA progression following endocrine therapy, serum sFas increased in parallel to the increase in PSA levels although the magnitude of change in sFas was very small. In patients whose tumor regressed following therapy, sFas levels did not change. sFas ligand levels were very low or below measurable levels in all specimens. sFas levels might be associated with poor prognosis in metastatic prostate cancer. Serum sFas ligand appears to have limited clinical relevance.