The Hall of Fame signal-caller is battling Squamous-cell carcinoma of the upper jawbone, and he is scheduled to undergo surgery Friday.
"This past couple of weeks has been difficult for me and because of the nature of social media I thought it would be best to share with everyone what has been going on with my health," Kelly said in a statement on the Bills' official website. "I was recently diagnosed with Squamous-cell carcinoma (cancer) of the upper jawbone.
"I have undergone tests which have shown that the cancer is isolated to my upper jaw and has not spread to other parts of my body. Surgery is scheduled for June 7th and doctors have told me that the prognosis for my recovery is very good."
Kelly knows there's a long road ahead, but he expressed optimism Monday that he'll be OK.
"When you hear the word cancer, it automatically scares the crap out of you," Kelly told reporters Monday. "I know it not only scared me, but it scared my family. Like everything, it's just another river to cross and another stumbling block.
"I've been to the top many, many times, and I've been to the bottom. It's just one of those roller-coaster rides I've been on throughout my life; it's just another challenge for me. I know I'll beat it -- that's the bottom line."
Our thoughts are with Jim and the entire Kelly family. All of us here at NFL.com and NFL Network wish him a speedy recovery.
Follow Marc Sessler on Twitter @MarcSesslerNFL.