This study analyzed the effects of dutasteride on lower urinary tract symptoms based on the association between changes in the total testosterone (TT)/dihydrotestosterone (DHT) levels and total prostate volume (TPV) reduction. Sixty participants diagnosed with benign prostatic hyperplasia were given 0.5 mg of dutasteride daily for 52 weeks. Measures of TT and DHT levels, TPV and uroflowmetry were obtained before and after dutasteride treatment. Forty-three patients demonstrated a TPV reduction of ≥5% (Group 1), whereas the remaining 17 patients demonstrated a TPV reduction of <5% (Group 2). DHT suppression and DHT/TT ratio at baseline were significantly higher in Group 1 than Group 2. International Prostate Symptom Scores (IPSS) and uroflowmetry were significantly improved in both groups. In Group 2, nine patients demonstrated some improvement in IPSS (Group 2A), whereas eight did not (Group 2B). The rate of TT increase and improvement in voiding symptoms were significantly higher in Group 2A than Group 2B. Dutasteride-induced TPV reduction is dependent on individual 5-α reductase inhibitor activity. Some patients demonstrating smaller dutasteride-induced TPV reduction may experience an improvement in voiding symptoms owing to an increased level of testosterone.