The announcement comes as the Boys -- who formed in 1993 -- are preparing for a nostalgia-filled summer tour with boy-band statesmen New Kids on the Block.
In a statement released to People, McLean said that his treatment would not impact the NKOTBSB tour. "With 2011 being a busy year for Backstreet, I want to be healthy and perform and sing for people at my very best and happiest," he wrote. "None of this will affect our current tour plans and I look forward to seeing everyone really soon!"
On Monday, McLean's Twitter account gave a hint that something was going on:
See u on the other side!
Today, New Kid Donnie Wahlberg tweeted his support:
As a friend of AJ McLean- what matters most is his health. As a partner in the band- nothing will stop all 9 of us from doing this TOGETHER!
Fans echoed the sentiment:
@DonnieWahlberg All I care is that he gets better for himself , family & friends but mostly for himself...AJ McLean will be in my prayers...
Last summer the Boys stopped by "Lopez Tonight." During the show they took questions from the Twitter audience, prompting A.J. to reveal another bad habit: He picks his fingers compulsively. "I'll do it until I bleed," he said. "I also twirl my shirt. That's how I don't pick my fingers."
Here's A.J. (and Howie, Brian, Nick and original BSB member Kevin) in happier days, performing at the 1997 MTV Europe Music Awards:
Surge Desk wishes A.J. the best and hopes Backstreet will be back, all right, very soon.
Follow Surge Desk on Twitter.