McLean has undergone treatment for addictions to drugs and alcohol in the past. Although the singer told People.com in 2008 he had been sober for six years, TMZ said he has since relapsed.
"Its gonna be a great year! I love u all and wish me luck!" McLean said via Twitter Sunday. "I start my sober journey tomm!"
The Backstreet Boys and its fellow 1990s boy band New Kids and the Block have been performing together in recent months.
"For personal reasons, I've checked myself into a rehab clinic," McLean said in a statement issued Thursday to the New York Daily News. "With 2011 being a busy year for Backstreet, I want to be healthy and at my best.
"Thank you for all of your support and respecting my family and friends privacy during this time. I'll see you really soon."
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