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Story by The Wall Street Journal

Oscar Munoz, the new chief executive of United Continental Holdings Inc., suffered a heart attack Thursday and is being treated at a Chicago hospital, people familiar with the matter said—the second major leadership shock in recent weeks for the world’s second-largest airline by traffic.

United directors “are waiting to hear back from his family and his doctors” about the severity of the heart attack, one of the people said Friday. They hope to find out the information within the next 24 hours, the person said, and then decide whether the big airline needs an interim leader.

United said in a brief statement Friday that it had been “informed by Oscar’s family that he was admitted to the hospital on Thursday and we will provide further details as appropriate. In the meantime, we are continuing to operate normally.”

United directors were informed about Mr. Munoz’s heart attack starting Thursday night, the person said. “It could be a mild heart attack, and he could be back in two weeks,” the person said. Depending on how long Mr. Munoz is disabled, “there are a few [United] executives who could be interim [CEO] for a short period of time,” this person added.

Mr. Munoz, 56 years old, was named as CEO on Sept. 8 following the abrupt ouster of Jeff Smisek, who had been CEO since the airline was formed by a merger in 2010. Mr. Munoz was preparing to host his first quarterly earnings call as CEO on Oct. 22.

United shares fell on Friday after The Wall Street Journal disclosed Mr. Munoz’s health problem, recently dropping 3.2% to $55.89.

Mr. Munoz, who is married with four children, said in a Sept. 30 interview with The Wall Street Journal that he has remained a keen surfer since his youth, and that he enjoys riding bikes and playing golf and tennis. He said he used to run marathons as well.

United has struggled with persistent delays, technical glitches and other operational difficulties, as well as poor labor relations, since the 2010 merger with Continental. Its business also has been clouded by a federal corruption probe into the Port Authority of New York and New Jersey that is examining the relationship of a former agency official with United. Mr. Smisek, along with two other senior executives, left as a result of an internal company investigation related to that federal probe, the company has said.

In the interview last month, Mr. Munoz acknowledged that the integration had been “rocky,” and that the “experience of our customers has not been what we want it to be.” He said he intended to address those problems, and that his initial focus as CEO has been meeting with United workers around the country for candid—and sometimes confrontational—discussions, seeking to win back the trust of United’s 85,000 employees, whose morale has suffered and who have become increasingly vocal in their complaints about management.

Mr. Munoz, a former CSX Corp. executive and longtime United board member, has been traveling extensively through United’s network to gather details on its operations. He was scheduled to lead a summit with labor leaders on Thursday, but the meeting was rescheduled, according to a spokesman for United’s pilots union, who said no reason had been given for the change.

Mr. Munoz said in the interview that he’d fielded emails from hundreds of United employees, often working into the early hours. Another of his initiatives was the launch of an online forum for United customers to raise issues and concerns about the airline.

United’s executive ranks have been diminished recently. In addition to Mr. Smisek and the two executives who resigned last month, Chief Financial Officer John Rainey left in August for PayPal.

Vice Chairman Jim Compton and acting CFO Gerry Laderman could be potential contenders if a temporary CEO is required. Both were veterans of Continental Airlines and worked there with Mr. Smisek before the merger with United in 2010.

Mr. Munoz told the Journal last month that he had initially said no when asked by United’s board to become CEO, largely because he worried his departure could cause problems for CSX, where he said an announcement of his promotion to become CEO of the railroad operator was imminent.

His eldest daughter also was getting married that weekend. He juggled the wedding and deliberations over his future through the Labor Day weekend, ultimately choosing to run United, partly because he wanted to return to a consumer-oriented business.

Mr. Munoz retains a home in Florida but planned to relocate his family to Chicago later.

The Association of Flight Attendants-CWA issued a statement Friday saying that its “members around the world are sending healing thoughts for Oscar Munoz. We offer support to his family and friends, and we await good news.”