Pretreatment serum level of testosterone (T) is a potential prognostic factor for prostate cancer. However, T levels in Japanese prostate cancer patients are unknown to date. To evaluate the clinical significance of pretreatment serum T level in such patients, serum T level was analyzed in relation to several clinical factors in a total of 130 patients with various stages of prostate cancer, 74 of whom had metastatic disease (stage D2) and received endocrine therapy as first-line treatment. The mean pretreatment T level in patients with non-metastatic prostate cancer (stages B + C) was significantly lower than that in stage D2 patients (B + C: 4.05 +/- 2.01 ng/ml; D2: 4.85 +/- 2.18 ng/ml, p = 0.0344). On the other hand, the mean serum level of T was higher in stage D2 patients who showed good response to endocrine therapy (CR: 5.42 +/- 1.55 ng/ml; non-CR: 4.30 +/- 2.63 ng/ml, p = 0.0320). When the 74 stage D2 patients were divided into high and low T level groups according to the median value, those patients with a high T level had significantly better cause-specific and progression-free survival. Multivariate analysis demonstrated that extent of bone metastases (EOD) grade, pretreatment serum T level and tumor marker response to endocrine therapy were significant predictors for progression-free survival. In conclusion, a higher pretreatment T level appears to be predictive of the marker response to endocrine therapy, showing positive prognostic value and indicating good prognosis in patients at the metastatic stage. However, a higher T level was also associated with stage progression of this disease.