Objectives: This study was performed to evaluate the effects of addition of tadalafil to tamsulosin in the treatment of brachytherapy patients with lower urinary tract symptoms (LUTS). Methods: Localized prostate cancer patients who developed LUTS after low-dose-rate brachytherapy (LDR-BT) were first treated with alpha-1 blockers (tamsulosin, 0.2 mg per day). Those still suffering from LUTS were additionally treated with phosphodiesterase type 5 inhibitor (tadalafil, 5 mg/day). LUTS was evaluated by the International Prostate Symptom Score (IPSS), IPSS Quality of Life (QOL) score, Overactive Bladder Symptom Score (OABSS), and the International Index of Erectile Function 5 (IIEF5) score before brachytherapy, before treatment with tadalafil, and after three months of treatment with tadalafil. Results: Twenty-three patients were enrolled in the study. Each subscore of the IPSS (storage symptom, voiding symptom, and postvoiding symptom) and the QOL score improved, but OABSS and IIEF5 did not show significant recovery after addition of tadalafil treatment. When patient characteristics and each question score were surveyed, time from seed implantation until treatment with tadalafil showed the strongest correlation with recovery. Conclusions: Addition of tadalafil to tamsulosin treatment was effective against LUTS in localised prostate cancer patients after brachytherapy. Evidence level: 2b