Background: The medium-to-long-term use of antimuscarinics alone or in combination with an a-blocker in men with an enlarged prostate is still controversial. This double-blind, placebo-controlled, randomized clinical trial aimed to investigate the efficacy and safety of medium-to-long-term use of tolterodine extended release (ER) with or without tamsulosin in patients with benign prostate hyperplasia (BPH) and larger prostate size. Methods: Totally, 152 patients (age >= 50 years) with BPH, International Prostate Symptom Score (IPSS) >= 12, quality-of-life (QoL) score >= 3, and total prostate volume >= 25 ml were enrolled in this study. The patients were randomized into four groups (n = 38 in each) to receive tolterodine ER placebo plus tamsulosin placebo, 0.2 mg tamsulosin plus tolterodine ER placebo, 4 mg tolterodine ER plus tamsulosin placebo, or tolterodine ER plus tamsulosin once daily for 24 weeks. IPSS (total, storage, and voiding subscales), QoL, maximum urinary flow rate (Q(max)), and postvoid residual volume (PVR) were collected at baseline, and at weeks 4, 12, and 24. Results: Compared with placebo, tolterodine ER plus tamsulosin significantly improved total IPSS (-7.15, -12.20, and -14.66 vs. -3.51, -5.78, and -7.23), storage IPSS (-3.56, -5.63, and -6.66 vs. -1.52, -1.21, and -2.43), voiding IPSS (-2.88, -5.10, and -6.48 vs. -1.52, -3.03, and -2.97), QoL (-1.21, -2.40, and -3.21 vs. -0.39, -1.41, and -1.60), Q(max) (2.21, 7.97, and 9.72 ml/s vs. 2.15, 2.44, and 2.73 ml/s), and PVR (-17.88, -26.97, and -27.89 ml vs. -12.03, -11.16, and -16.73 ml) at weeks 4, 12, and 24, respectively; the differences were all statistically significant (P0.05). However, a gradual increase in PVR (10.03, 10.41, and 12.89 ml) and more urinary AEs suggestive of urinary retention (11/38 vs. 4/38) were observed. Conclusion: Medium-to-long-term use of tolterodine ER plus tamsulosin should be recommended in patients with BPH and an enlarged prostate volume.