Goldman Sachs Group Inc. Chief Executive Lloyd Blankfein said he is suffering from a curable form of blood cancer, raising anew the sensitive issue of who may one day replace him.

Mr. Blankfein, 61 years old, said he would remain at the top of the firm as he undergoes chemotherapy treatment for lymphoma over the next few months. He said he would curtail travel but otherwise continue his duties at Goldman, where he has served as CEO for nine years.

Yet...