In each season from 2004 through 2008, he hit at least 40 home runs, and 38 apiece in 2009 and 2010. Even as a Reds rookie in 2001, he hit 19 home runs in 66 games. In 122 games last season, he hit 11.
This season, at age 31, Dunn's numbers have rebounded, although his batting average is still below his .243 career average. His slugging and OPS percentages are higher than his career numbers. And he's on a pace to once again hit 40-plus home runs.
Yes, he's leading the league in strikeouts, but he's also high on the chart for walks. And his career home runs, runs scored and runs batted in put him in the company of Hall of Famers Reggie Jackson and Harmon Killebrew, who had similar numbers when they were 31.
So, what went so wrong last season?
"I don't know," said Dunn, a right-handed fielder and left-handed batter. "If I knew, I would've fixed it. I tried everything other than hitting right-handed. I just couldn't get out of it."
Well then, what happened this season?
"Nothing," Dunn said with a shrug. "I'm the same guy I was before."
When Dunn and his woeful 2011 stats arrived in Spring Training this year, Jeff Manto was the new White Sox hitting coach. He had the good sense to do, well, almost nothing to improve Dunn's numbers.
"No tinkering," Manto said. "Not at all. What I did with Adam is just reassure him that he's a great Major League hitter. Just look at his numbers. The numbers indicate that he's a great power hitter. I just reminded him of that and stayed out of his way."
It's probably a good thing that Dunn, a star quarterback in high school, ultimately settled on a career in baseball. When Cincinnati drafted him in the second round in 1998, he already had signed a letter of intent to play football at the University of Texas. The deal he reached with the Reds allowed him to play football for the Longhorns and Minor League baseball in the summer.
Dunn was a redshirt backup quarterback to Major Applewhite his first year. Then Chris Simms, son of former Giants quarterback Phil Simms, committed to Texas.
"Things weren't going kind of the right way, not the way I envisioned them," Dunn said.
"(Coach Mack Brown) basically said, 'You can do this at quarterback or you can try a different position,' and I said, 'I'll go try a different occupation.' I knew I'd have to give up one eventually, and it seemed like a sign to me."
By July 2001, he was in the Majors. The Reds traded him to Arizona in August 2008.
"Obviously, they were starting to go younger, kind of rebuilding, I would guess," Dunn said. "I knew that was going to happen. No hard feelings there."
He left the D-backs after the season, signed a two-year contract with the Nationals, then departed for the White Sox and the four-year contract that began with his crash-and-burn 2011 season.
"I only watched him from afar before, so I don't know what went wrong," Manto said. "What I'm seeing right now is somebody who is motivated, I'm seeing somebody who has put last year behind him and is moving forward."
Bruce Lowitt is a freelance writer based in Tampa, Fla.