Dr. Wolchok said the problem had occurred in one patient at Sloan Kettering but had cleared up on its own. He agreed that it was advisable to order extra heart tests for patients taking checkpoint combinations.

Dr. Benjamin A. Olenchock, a study author from the Division of Cardiovascular Medicine at Brigham and Women’s Hospital in Boston, was not available for an interview but said in a written statement that the heart problem had affected patients at his hospital. “As the number of patients treated with checkpoint inhibitors has markedly increased, rare cases of cardiac toxicity associated with the use of these cancer therapeutics, sometimes resulting in death, have been seen at multiple institutions including our own,” the statement said.

The first checkpoint inhibitor was approved in 2011. The drugs work by unleashing T-cells, a type of white blood cell, to kill cancer. But sometimes, the T-cells go into hyperdrive and attack healthy tissue. Doctors have known for years that the drugs can have dangerous side effects, including gut, lung and thyroid trouble. But the cardiac problems have taken longer to emerge.

There have been scattered reports in other, less prominent medical journals of heart problems, some fatal, in small numbers of patients taking checkpoint inhibitors alone or in combination. The new report is the most in-depth analysis, including tests for possible genetic or viral causes (none were found) and an examination of a drug-company database to identify other cases.

The patients described in Dr. Moslehi’s article — a woman, 65, and a man, 63 — developed heart problems and died a few weeks after just one intravenous treatment with a combination of two checkpoint inhibitors: Opdivo and Yervoy. Both patients had advanced melanoma, a deadly skin cancer, and were enrolled in studies. Neither had a history of heart disease.

The woman had chest pains, shortness of breath and fatigue, and was admitted to the hospital 12 days after her first dose of the drugs. She had myocarditis — inflammation of the heart — as well as other inflamed muscles and abnormal heart rhythms.