Objective: Rhenium-188-HEDP is an effective radiopharmaceutical for the treatment of painful bone metastases from prostate cancer. The effectiveness of the β-radiation emitted by 188Re might be enhanced by combination with chemotherapy, using the radiosensitization concept. Therefore, the authors investigated the combined treatment of the taxanes, docetaxel and cabazitaxel, with 188Re in prostate carcinoma cell lines. Materials and Methods: The cytotoxic effects of single and combined treatment with taxanes and 188Re were investigated in three human prostate carcinoma cell lines (PC-3, DU 145, and LNCaP), using the colony-forming assay. The half maximal effective concentration (EC50) of all individual agents was determined. The combined treatment was studied at 0.25, 0.5, 1, 2, and 4 times the EC50 of each agent. The interaction was investigated with a regression model. Results: The survival curves showed dose-dependent cell growth inhibition for both the taxanes and 188Re. The regression model showed a good capability of explaining the data. It proved additivity in all combination experiments and confirmed a general trend to a slight subadditive effect. Conclusions: This proof-of-mechanism study exploring radiosensitization by combining 188Re and taxanes showed no synergism, but significant additivity. This encourages the design of in vivo studies. Future research should explore the potential added value of concomitant treatment of bone metastases with chemotherapy and 188Re-HEDP.