Hosmer was the first national finalist in the Pitch, Hit & Run contest to reach the Major Leagues. He competed in 1998 at the All-Star Game at Denver.
Major League Baseball authenticators were on "high alert" to document: His game-worn uniform, Royals lineup card, all three bases after they come off the field, his bat and other items. They did the same type of thing for pitcher Stephen Strasburg's debut with Washington.
Hosmer is the fourth youngest player in the Majors and the second youngest in the American League. The Cubs' Starlin Castro is the youngest, born on March 24, 1990. Hosmer was born Oct. 24, 1989.
He stepped into the batter's box for the first time at 7:34 p.m. CT, smoothed the dirt and took a practice swing. Gio Gonzalez's first pitch was a 96-mph fastball called ball one.
The attendance for his debut was 30,690, third largest of the season for the Royals. The game was their shortest (2:22) of nine innings this year.