For three years, he put in countless hours at the gym, bulking up from 155 to 178 pounds, and his game improved, too.
He got his first PGA TOUR victory in 2006 and had three top-10 finishes this year, giving him confidence going into the Masters. Then, two nights before the tournament began, he started getting stomach cramps.
Two trips to the hospital, a visit to a specialist and a month later, the parasite that caused him to lose 22 pounds is gone.
Immelman is healthy again -- and so is his game.
In his first event since he finished tied for 55th at Augusta while eating only toast for four days, Immelman shot a 4-under 68 in the opening round of the Wachovia Championship on Thursday, leaving him near the top of the leaderboard.
'Mentally, I stayed very calm,' Immelman said. 'Because I said to myself, 'How much do you actually expect right now? Just take it easy.' Then I started rolling a few putts in, and I really started playing solidly.'
The 27-year-old Immelman recovered after bogeying two of the first three holes, reeling off four straight birdies on the front nine at Quail Hollow. He finished par-par-birdie on what's rated the toughest closing stretch on the PGA TOUR the past three years.
All from a golfer who didn't pick up a club after the Masters' final day until last week.
'I feel fantastic. I just don't feel like I have as much energy as I used to,' said Immelman, who added his iron shots weren't traveling as far. 'It's part of just getting the weight back.'
And that peeves the 5-foot-9 South African.
'It took me three years to get from 155 to 178, and my goal has always been 180,' he said. 'It's incredible I can lose three years of work in three weeks. It kind of (ticks) me off.'
It also scared him. Doctors weren't sure what he had or how he got it. Just when he started to feel good, he had a relapse and was forced to withdraw from the Verizon Heritage and Byron Nelson Championship.
Immelman eventually saw a specialist, who determined he had contracted a parasite and then a stomach infection. Told to fast for three days, then eat only rice for two more, Immelman got the go-ahead to play Tuesday. Though his doctor did warn him, 'just try to stay lactose-free.'
Immelman wanted to play the Wachovia, where last year he three-putted the 72nd hole for bogey, then lost on the first playoff hole to Jim Furyk.
'The golf course is playing very difficult, just like a major. So you've got to be on your game,' Immelman said. 'I really didn't know what to expect of myself having not competed for four weeks. But there are definitely some good vibes for me around here.'