AbstractPlasma fucosyltransferase activity was evaluated as an indicator of an impending blastic transformation in 25 patients with chronic granulocytic leukemia (CGL). Fifteen age‐and sex‐matched controls were also studied. The level of enzyme activity was significantly higher in the plasma of patients with blastic transformation (1,630 ± 570 units) compared with steady state chronic granulocytic leukemia (509 ± 110 units) and normal controls (354 ± 57 units). In three patients with CGL, a rise in fucosyltransferase activity preceded any other clinical or laboratory parameter of blastic transformation by 16–