OBJECTIVES: To identify the potential influence of antimuscarinics on morphometric parameters of the prostate in patients with benign prostatic enlargement and overactive bladder.METHODS: Non-neurological patients with prostate volume >30 mL, predominately storage lower urinary tract symptoms, three or more urgency episodes per 24 h, maximum flow rate ≥10 mL/s and post-void residual ≤100 mL were recruited for this study. They were randomized to receive either tamsulosin or tamsulosin + solifenacin. Patients were submitted to transrectal and transvesical ultrasonography, pressure-flow study and prostate-specific antigen test, and completed the International Prostate Symptom Score, bladder diary and overactive bladder questionnaire at induction and at 6 months. End-study changes in morphometric prostate parameters (total prostate and adenoma volumes, prostate vascularity), as measured by transrectal ultrasound, were the principal outcomes.RESULTS: A reduction in total prostate volume (mean -9.5%) was noted in the combination group, as opposed to an increase in the monotherapy group (+9.2%; P < 0.001). Similar changes were reflected in adenoma volume (monotherapy +17.4% vs combination -12.5%, P = 0.001) and in prostate vascularity (monotherapy +149.3% vs combination -19.8%, P = 0.001). Both treatment regimens improved the International Prostate Symptom Score (P = 0.001); monotherapy improved the voiding subscale (P = 0.01) more, whereas combination therapy improved the storage subscale (P = 0.024). Cystometric capacity improved in the combination group (P < 0.001). Post-void residual was increased in the combination group (+34.79%), as opposed to a decrease in the monotherapy group (-17.05%; P = 0.001).CONCLUSIONS: The results of this pilot study suggest that solifenacin might affect morphometric properties of the prostate, decreasing total prostate and adenoma volume, as well as vascularity. A molecular effect of antimuscarinics on the prostate, in parallel with their expected bladder effect, warrants further investigation.