On 25 November 2009, Dio's wife and manager announced that he was diagnosed with stomach cancer:

"Ronnie has been diagnosed with the early stages of stomach cancer. We are starting treatment immediately at the Mayo Clinic. After he kills this dragon, Ronnie will be back on stage, where he belongs, doing what he loves best, performing for his fans. Long live rock and roll, long live Ronnie James Dio. Thanks to all the friends and fans from all over the world that have sent well wishes. This has really helped to keep his spirit up." -- "He has had a few hiccups between Christmas and New Year's," she said in a statement to fans. "He has had a blood clot, a trip to the emergency room, and a three-day stay at the hospital."

ArtisanNewsService — April 13, 2010 — "One of heavy metal's premiere vocalists Ronnie James Dio shares his thoughts on his battle with stomach cancer at the Revolver Golden Gods awards."

On 14 March 2010, Dio's wife and manager Wendy posted an online update on his condition:

"It has been Ronnie's 7th chemo, another cat scan and another endoscopy, and the results are good - the main tumour has shrunk considerably, and our visits to Houston (MD Anderson Cancer Center in Texas) are now every three weeks instead of every two weeks."

References:
Ronnie James Dio. Wikipedia.
http://www.nme.com/news/black-sabbath/49346
Black Sabbath, Dio singer Ronnie James Dio gives cancer update. NME.com.