By Emily Glazer 

J.P. Morgan Chase & Co. Chairman and Chief Executive James Dimon discovered he had throat cancer after noticing something was amiss with his health within the last few weeks, according to a person close to the bank executive.

Mr. Dimon, who runs the nation's largest bank by assets, disclosed Tuesday in a memo to shareholders and employees that his cancer is "curable" and hasn't spread to other areas. He will spend this summer undergoing chemotherapy and radiation treatments that will cut into his travel schedule, the company said.

Mr. Dimon, 58 years old, went for initial medical tests within the last few weeks, the person close to him said. Last week, he underwent a number of exhaustive tests, including a CT scan, PET scan and biopsy, this person added.

As Mr. Dimon got more reliable and solid medical information about his condition, he called J.P. Morgan's lead board director Lee Raymond in addition to a few other directors over the past week, people close to Mr. Dimon said, declining to name the other board members.

Mr. Raymond, former Exxon Mobil Corp. chairman and CEO, and the other directors then connected with other board members. Mr. Dimon has been in close contact with Mr. Raymond and some other directors over the past week, these people said.

Mr. Dimon still plans to take part in J.P. Morgan's earnings call on July 15, barring any treatment conflict, these people said.

While no one will take over for Mr. Dimon during his treatment, the unexpected health issue calls into question the company's succession plan, should Mr. Dimon need to step back more than expected to undergo treatment.

The bank's board has had succession plans in place well before Mr. Dimon's throat cancer became public Tuesday evening, according to the company.

The bank's retail head Gordon Smith and asset-management head Mary Erdoes are two of the top candidates to take on bigger roles if an immediate change in leadership is necessary, a person familiar with the bank's plans said.

However, some industry experts have said both those candidates only have experience within their respective units and aren't likely permanent successors to Mr. Dimon unless they are shifted to other leadership roles elsewhere within the bank.

Mr. Dimon has indicated in recent months that he plans to stay at the company for several years. Longer-term succession candidates include Chief Operating Officer Matt Zames; Doug Petno, chief of the commercial bank; Daniel Pinto, chief of the corporate and investment bank; Marianne Lake, chief financial officer; and Ashley Bacon, chief risk officer, the person familiar with the matter said.

Another candidate, former investment-bank executive Michael Cavanagh, left J.P. Morgan to become co-chief operating officer at investment firm Carlyle in March.

The succession planning may ultimately prove premature. Mr. Dimon plans to work during his treatment and take his usual August vacation while undergoing chemotherapy, which is typically more physically debilitating, a company spokesman said.

Current and former employees say they didn't notice anything unusual with Mr. Dimon this week. On Monday night, he gave remarks at a retirement party for a longtime employee and seemed to be in good health and high spirits, a person who attended the event said.

J.P. Morgan employees reached since Tuesday say they are surprised but hopeful that Mr. Dimon will make a full recovery.

Write to Emily Glazer at emily.glazer@wsj.com

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