PURPOSE We studied the relationship of nocturia with the International Prostate Symptom Score in men with lower urinary tract symptoms due to benign prostatic hyperplasia (BPH). MATERIALS AND METHODS Baseline symptom score and the change in the score by treatment were analyzed in 219 consecutive Japanese men with BPH using basic statistics, and correlation, cluster and principal component analyses. RESULTS The average nocturia score was the middle of that of the 7 symptoms at baseline and by far the highest after treatment. Hence, the score change was the smallest. Analysis of baseline scores demonstrated that all symptoms were the initial component in principal component analysis with individual symptom scores correlating with the total symptom score and quality of life index. However, the correlation was least for nocturia. Observations were similar when analyzing the score change by treatment. These results indicate that the nocturia score is least specific to symptoms associated with BPH or least sensitive to the therapeutic effect on symptoms. This finding may be related to the high nocturia score in the age matched control population. CONCLUSIONS The nocturia score of the International Prostate Symptom Score behaves differently in the symptom complex of BPH in Japanese men, partially because it is most confounded by the aging factor.