Studies were performed on a teen age female identical twin with cyclic leukocytosis and Ph1 positive chronic myelogenous leukemia (CML), in an attempt to analyze the relationship of these cycles and to utilize the cycles as reference points for kinetic studies and as landmarks in the progression of her disease. She remained untreated for 2.5 yr. Colony stimulating factor levels showed an inverse relationship to the peripheral leukocyte count. Two distinct phases in cyclic patterns of peripheral leukocyte counts prior to treatment could be appreciated in retrospect: phase 1, a stable 15 mth period during which leukocyte counts peaked every 10 wk and then returned to baseline levels, and phase 2, an 18 mth period of insidious deterioration during which the low points of successive cycles became progressively higher and leukocytes were accumulating in the peripheral blood at a rate of 400 X 106/day. The authors propose that the increased marrow production of myeloid cells in this disease does not represent a life threatening situation until cell accumulation begins. They attempted, therefore, to halt or to reverse this accumulation by cell separator leukapheresis.