The purpose of this study was to clarify the biological maturity of top junior baseball players in relation to Peak height velocity (PHV) ages and birth month. The subjects were 25 junior high schools (TOP) with high competition level and 30 standard baseball players （BB）. Using the proportional distribution method, the PHV age was calculated as an index of biological maturity. Consequently, the PHV age of TOP group （12.3± 0.7 years） was significantly lower than the BB group （13.0±0.8 years）. The birth month distribution was different between the two groups. 84％ of the TOP group and 56％ of BB group were born between April and December, suggesting that TOP group matures earlier than the BB group by 0.9 years.
