A 46-year-old man with blastic crisis of chronic myelogenous leukemia was given 950 rads whole-body irradiation followed immediately by 17.6 x 109 marrow cells. The marrow donor was the patient's sister who matched the patient in 11 of 12 HL-A groups and whose leukocytes reacted to the patient's leukemic cells in mixed leukocyte culture. The patient's white blood cell count began to go up on the 13th day and marrow engraftment was confirmed by repeated cytogenetic analyses. Secondary disease with skin rash and diarrhea was evident by the end of the second week but appeared to be controlled by methotrexate. The patient then developed fever, recurrent diarrhea and pneumonitis and died 56 days after irradiation. Autopsy showed inclusion bodies typical of cytomegalovirus. There was no histological evidence of acute secondary disease nor of leukemia.