We evaluated the association of circulating levels of heat shock protein 70 (Hsp70) in plasma with clinical behavior and progression in 139 chronic myeloid leukemia (CML) patients. Circulating Hsp70 levels did not differ significantly between CML patients in the chronic phase (n=93; median 33.24 ng/mL, range 3.89-128.2 ng/mL) and those in the accelerated/blast phase (n=46; median 26.57 ng/mL, range 4.5-114.7 ng/mL). However, overall CML patients had significantly higher levels of Hsp70 than healthy subjects (n=95, median 4.17 ng/mL, range 1.75-24.7 ng/mL) (P<0.001). In chronic phase CML patients, Hsp70 levels above the median were associated with a higher rate of progression to the accelerated/blast phase and a tendency toward shorter survival. Plasma Hsp70 thus could be a potential marker for predicting disease progression in patients with chronic phase CML.