Background: Carboplatin plus docetaxel (CD) may be effective in mDRPC. Platinum(II)-complexes interfere with steroid biosynthesis and thus lower testosterone levels. In this study, the impact of CD on free and total testosterone (fT, TT) serum levels and the prognostic role of fT and TT were analyzed in mDRPC patients. Methods: Docetaxel failure/resistance was defined according to the Prostate Cancer Working Group (PCWG2 2007) criteria. Treatment consisted of at least 2 cycles of carboplatin AUC5 iv for 30 min on day 1 every 4 weeks (q4w), docetaxel (35 mg/m2) iv for one hour on days 1, 8, (15) plus prednisone 2x5 mg/day orally after receiving informed consent until disease progression or occurrence of intolerable adverse effects. Efficacy measures were done following PCWG2 recommendations. Results: Of the 118 pts. treated since February 2005, 95.8% had bone, 47.5% lymph node, 28.0% liver and 20.3% lung metastases. Median follow-up time was 14.4 months at the time of the current analysis. The objective response rate (ORR) was 46.4% in the 69 pts. with measureable disease (58.5%). Response of prostate-specific antigen (PSAR ≥ 50%) was observed in 56 (47.9%) patients. Median progression free survival (PFS) was 7.6 months (CI 95% 6.0, 9.1) and median overall survival (OS) was 15.7 months (CI 95% 12.2, 19.2). The most common reversible grade 3/4 toxicity was leukopenia/ neutropenia (36.4/28.8%). Median FT serum levels were 0.35 pg/mL before and < 0.18 pg/mL during CD treatment (nadir levels, p < 0.001; detection limit < 0.18 pg/mL). In multivariate analyses, LDH > 2xULN, PSAR≥50% and FT nadir levels below the detection limit ( < 0.18 pg/mL) during CD treatment were associated with longer PFS (p < 0.05). Conclusions: These data suggest that CD may be an important salvage treatment option for mDRPC patients by inhibition of the testosterone biosynthesis.