Thrombelastographic evidence of hypercoagulability, including shortening of r-time (P < 0.01); shortening of k-time (P < 0.01); and widening of trace angle (P < 0.01) were observed in a group of 21 Chinese surgical patients when (a) the amount of blood loss was at an estimated 10% of total blood volume and (b) the amount of blood loss was at an estimated 15% of blood volume. The amount of blood loss was documented by haemoglobin measurements. No evidence of hypercoagulability was observed at around one hour into the operation in the absence of bleeding. We conclude that a mild to moderate degree of surgical blood loss with haemodilution is associated with the development of hypercoagulability as measured by thrombelastography. Further studies looking at the thrombebolic outcome in such groups of patients is warranted. It is also suggested that caution should be exercised in the use of intraoperative isovolaemic haemodilution until the phenomenon is further investigated.