Fifty-six of 195 Ph1-positive patients with chronic granulocytic leukemia were found to have Ph1-negative metaphases in marrow aspirates on one or more occasions. In 22 cases, Ph1-negative cells were found prior to initiation of antileukemic therapy. Five patients were in the blastic stage of the disease when Ph1-negative mitoses were seen. The finding of Ph1-negative cells appeared to be related principally to short duration of CGL and to administration of antileukemic therapy (conventional agents and doses, in most cases). Ph1-negative cells were usually not found more than 2 yr after the diagnosis of leukemia, but in a few cases, they were seen as long as 5–10 yr after diagnosis. Only a minority of metaphases analyzed were Ph1-negative, except in the case of 6 patients who transiently had 50% or more Ph1-negative cells after antileukemic therapy. The presence of Ph1-negative cells in marrow was not associated with any survival advantage in this series.