PURPOSE: Many patients with benign prostatic hyperplasia (BPH) have storage symptoms. The aim of this study was to evaluate the effects of treatment with a 5-alpha reductase inhibitor (5ARI) on storage symptoms in patients with BPH. METHODS: This study was conducted in 738 patients with lower urinary tract symptoms secondary to BPH. Patients with a prostate volume of higher than 30 mL on the transrectal ultrasound were classified into two groups: group A, in which an alpha blocker was solely administered for at least 12 months, and group B, in which a combination treatment regimen of an alpha blocker plus 5ARI was used. This was followed by an analysis of the changes in parameters such as the total International Prostate Symptom Score (IPSS), voiding symptom subscore, and storage symptom subscore between the two groups. In addition, we examined whether there was a significant difference between the two groups in the degree of change in storage symptoms between before and after the pharmacological treatment. RESULTS: Of the 738 men, 331 had a prostate volume > or =30 mL, including 150 patients in group A and 181 patients in group B. Total IPSS, the voiding symptom subscore, and the storage symptom subscore were significantly lower after treatment than before treatment in both groups (P<0.05). A comparison of the degree of change between before and after treatment, however, showed no significant differences in the storage symptom subscore between the two groups (P>0.05). CONCLUSIONS: Alpha blocker and 5ARI combination treatment is effective for patients with BPH including storage symptoms. However, 5ARI does not exert a significant effect on storage symptoms in BPH patients.