Duffy, 21, was the Royals' representative in the 2009 Futures All-Star Game last year at St. Louis and pitched two-thirds scoreless innings. Last season, Duffy made 24 starts for Class A Wilmington and had a 9-3 record with a 2.98 ERA.
He was one of the Royals' biggest hopes to develop into a premier left-handed starter.
However, on Tuesday Duffy told J.J. Picollo, the Royals' assistant general manager of scouting and player development, that he was leaving the Minor League camp.
"Danny informed us yesterday that his desire is to leave baseball at this time to reassess his life priorities. Should Danny decide in the future to return to the game, we will be happy at that time to discuss the possibility," Picollo said in a statement.
Duffy was invited to the Major League camp as a non-roster player to gain experience in the workouts. He did not pitch in any Cactus League games and was shut down early in March because of stiffness in his left elbow and wasn't likely to pitch for Double-A Northwest Arkansas until May.
But Picollo indicated that medical issues were not the reason for Duffy's decision.
"It's disappointing," Picollo said. "He just felt like he needed some time away from the game. We're supportive of it and we've got to just work with him through this."
Last season, Duffy was a Carolina League All-Star and, in addition to a sub-3.00 ERA, he ran up 125 strikeouts in 126 2/3 innings against just 41 walks. He held opponents to a .230 average.
Duffy, from Carillo High School in Lompoc, Calif., was selected by the Royals in the third round of the 2007 First-Year Player Draft. He broke in that year with Surprise of the Arizona Rookie League and was 2-3 with a 1.45 ERA in 11 games. Big stat: 63 strikeouts in 37 1/3 innings.
In 2008, he was Class A Burlington's Pitcher of the Year when he went 8-4 with a 2.20 ERA. In his last 11 starts, Duffy was 6-1 and had a 0.99 ERA as Burlington won the Midwest League title.
"He has a plus-fastball, a curveball and a changeup -- basic pitches but a great competitor and he had a lot of success going on in his career here," Picollo said. "He has what you're looking for in a pitcher."
His combined record in three Minor League seasons: 19-10, 2.49 ERA in 52 games with 290 strikeouts and just 83 walks in 245 2/3 innings.
Duffy's action sparked memories of pitcher Zack Greinke's departure from the Royals' big league camp in 2006. Later it was learned that Greinke had social anxiety disorder, which was successfully treated. He returned that year and began an ascent that led to the 2009 American League Cy Young Award.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.