Abnormal hematocrit (Hct) is associated with an increased risk of pre-hypertension and all-cause death in general pop-ulation,and people with a high Hct value are susceptible to arterial cardiovascular disease and venous thromboembolism.In this study,we report for the first time on the ability of thermoacoustic imaging (TAI) for in vivo evaluating Hct changes in human forearms.In vitro blood samples with different Hct values from healthy volunteers (n =3) were prepared after centrifugation.TAI was performed using these samples in comparison with the direct measurements of conductivity.In vivo TAI was conducted in the forearm of healthy volunteers (n =7) where Hct changes were produced through a vascular occlusion stimulation over a period of time.The results of in vitro blood samples obtained from the 3 healthy subjects show that the thermoacoustic (TA) signals changes due to the variation of blood conductivity are closely related to the changes in Hct.In addition,the in vivo TA signals obtained from the 7 healthy subjects consistently increase in the artery/muscle and decrease in the vein during venous or arterial occlusion because of the changed Hct value in their forearms.These findings suggest that TAI has the potential to become a new tool for monitoring Hct changes for a variety of pre-clinical and clinical applications.