Background/Aim: Reference values for penile length have not been established for Japanese boys. We aimed to develop percentiles and means (and standard deviations) of stretched penile length (SPL) in Japanese boys. Methods: A cross-sectional study was performed in 1,628 Japanese boys aged <9 years from 2007 through 2014. The LMS method was used to develop a growth reference and chart for SPL in boys aged 0-7 years. Interobserver variation in SPLs was assessed in 32 boys (aged 0-11 years, median 3 years) using the Bland-Altman plot. The correlation between SPL and stature, weight or body mass index (BMI) was analyzed by the Pearson test. Results: SPL increases continuously during the prepubertal period, and most rapidly in the first 4 months of life. No significant fixed or proportional bias was found for interobserver variation (p = 0.5; r = 0.33, p = 0.06). There was no significant correlation between SPL z-score and stature z-score (r = 0.14, p < 0.001), weight z-score (r = 0.09, p < 0.001) or BMI z-score (r = 0.01, p = 0.71). Conclusion: These data serve as an updated growth reference for SPL in Japanese boys aged 0-7 years.
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