Androgen deprivation therapy (ADT) is the standard of care for patients with metastatic prostate cancer. However, whether serum testosterone levels, using a cut?off point of 50 ng dl?1, are related to the effective time of ADT in newly diagnosed prostate cancer patients remains controversial. Moreover, recent studies have shown that some patients may beneift from the addition of upfront docetaxel chemotherapy. To date, no studies have been able to distinguish patients who will beneift from the combination of ADT and docetaxel chemotherapy. This study included 206 patients who were diagnosed with metastatic prostate cancer and showed progression to castrate?resistance prostate cancer (CRPC). Serum testosterone levels were measured prospectively after ADT for 1, 3, and 6 months. The endpoint was the time to CRPC. In univariate and multivariate analyses, testosterone levels<50 ng dl?1 were not associated with the effective time of ADT. Receiver operating characteristic and univariate analysis showed that testosterone levels of≤25 ng dl?1 after the ifrst month of ADT offered the best overall sensitivity and speciifcity for prediction of a longer time to CRPC (adjusted hazard ratio [HR], 1.46; 95% conifdence interval [95% CI], 1.08–1.96;P= 0.013). Our results show that serum testosterone level of 25 ng dl?1 plays a prognostic role in prostate cancer patients receiving ADT. A testosterone value of 25 ng dl?1 after the ifrst month of ADT can distinguish patients who beneift from ADT effectiveness for only a short time. These patients may need to receive ADT and concurrent docetaxel chemotherapy.