The Backstreet Boys singer - who has previously been treated for depression, anxiety and excessive consumption of alcohol - voluntarily checkedhimselfinto a treatment clinic on 10.01.11 after realising he needed help, but insisted it would not affect the group.
In a statement he said: "For personal reasons, I`ve checked myself into a rehab clinic. 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.
"None of this will affect our current tour plans and I look forward to seeing everyone really soon! Thank you for all of your support and respecting my family and friends privacy during this time."
A.J. - real name Alexander James - has previously dropped hints he was struggling with alcohol issues, writing on his twitter account @skulleerozon: "Thank u all for my bday wishes and tweets:)! Its gonna be a great year! I love u all and wish me luck! I start my sober journey tomm! Ttfn! (sic)"
He added in a post: "Hey all wish me luck! This is gonna be an amazing year for me and all around me! I love u all and appreciate all the love and support.
"See u on the other side!(sic)"
A.J`s bandmates - Brian Littrell, Nick Carter and Howie Dorough - tweeted their support for him, saying: "we love and support our brother @skulleeroz. We are proud of him and thank all of our fans for the support. We love u!"
Backstreet Boys are currently planning a summer 2011 tour with New Kids on the Block.