"After 40 years of life dedicated to AC/DC, guitarist and founding member Malcolm Young is taking a break from the band due to ill health," the group said on its website, without disclosing the nature of his illness.
"Malcolm would like to thank the group's diehard legions of fans worldwide for their never-ending love and support."
The statement added: "The band will continue to make music."
Formed in Australia in 1973 by Malcolm and brother Angus Young, AC/DC is famed for rock anthems Let There Be Rock, Whole Lotta Rosie and Highway to Hell.
Rumours about the band's future began circulating earlier this week following an anonymous tip-off to a Perth radio station by someone calling themselves Thunderstruck. The tipster said in an e-mail that Young, 61, was "very ill" and had returned with his family to Australia.
"That is true, Malcolm is sick," Mark Gable, the frontman for Australian band Choirboys and a former record label mate, told the ABC on Wednesday. "From what I understand, and it's even been confirmed in part by his son Ross, that it would appear Malcolm is unable to perform any more."
The band appealed for Young's privacy to be respected and its website was full of comments and prayers for the guitarist and his family.
"If you return we will be here waiting," said one message signed "xo".
Reports in Australia said AC/DC, which has sold more than 200 million albums, would still keep a May date at a Vancouver recording studio.
The band is known for its blues-tinged rock 'n' roll, pounding sound and the raw vocals of Briton Brian Johnson, who took over when singer Bon Scott died after a night of heavy drinking in 1980.