It seemed to be no big deal when Gary D. Cohn, the president of Goldman Sachs, spoke at a panel event Monday night.

But Mr. Cohn, who had just flown in from Singapore, was appearing on short notice. Unbeknown to the audience, he was filling in because his boss, Lloyd C. Blankfein, had hours earlier learned he had cancer.

Mr. Blankfein, who is Goldman’s chairman and chief executive, publicly revealed Tuesday morning on the firm’s website that he had what he described as a “highly curable” form of lymphoma.

The 61-year-old Mr. Blankfein will remain in charge as he undergoes chemotherapy over the next several months. “My doctors have advised me that during the treatment, I will be able to work substantially as normal, leading the firm,” he said in a statement posted on the firm’s website on Tuesday.