JPMorgan Chase & Co chief executive Jamie Dimon said he has been diagnosed with throat cancer but that the ailment is curable.
Mr Dimon said he plans to remain actively involved in the business and continue to run the largest bank in the US.
In a memo to the bank's employees and shareholders yesterday, Dimon, 58, said the cancer was caught at an early stage and is confined to the original site and the adjacent lymph nodes on the right side of his neck.
He will undergo radiation and chemotherapy treatment that is expected to last about eight weeks.
"I feel very good now and will let all of you know if my health situation changes," Dimon wrote in the memo. "Importantly, there is no evidence of cancer elsewhere in my body."
The company has a short, medium and long-term succession plan in place, a source at the bank said.
The treatment will curtail Mr Dimon's travel during the period. He was expected to travel to Europe this week, where he was scheduled to meet with the prime ministers of Greece and Italy and also visit Spain, England and Germany. But those plans have been cancelled.
The news of Dimon's ailment is an unexpected blow for JPMorgan. Dimon, who has been chief executive of the bank for the past eight years, was credited for steering the bank successfully through the financial crisis.
In recent years, however, the bank has run into headwinds that have taken off some of its sheen.
In 2012, the bank lost $6.2 billion from risky derivatives bets that came to be known as the "London Whale" trades. And late last year, it agreed to a $13 billion settlement with the US government to settle charges that it overstated the quality of mortgages it was selling to investors in the run-up to the financial crisis.
The bank's massive London Whale loss prompted shareholders last year to push Dimon to give up his chairman title. But shareholders backed off when Dimon's allies suggested at the time he might leave the company if he lost one of the titles.