We examined the effectiveness of supplemental administration of flavoxate hydrochloride in patients with benign prostatic hyperplasia (BPH) whose nocturia was not adequately relieved by an alpha1-adrenoceptor blocker. Fifty-two patients who had two or more nocturnal micturition after administration of tamsulosin hydrochloride or naftopidil for 4 weeks or more received 400-600 mg of flavoxate hydrochloride in addition to an alpha1-adrenoceptor blocker for another 8-12 weeks. With supplemental administration of flavoxate hydrochloride, significant improvement was observed in the number of nocturnal micturition, total International Prostate Sympton Score, quality of life score and BPH impact index. No significant change was observed in the voided volume, Qmax, voiding time and residual urine volume. Supplemental administration of flavoxate hydrochloride is therefore effective for the improvement of nocturia and QOL in BPH patients resistant to an alpha1-adrenoceptor blocker.