The blood viscosity of 21 patients with esophageal or cardiac carcinoma during preoperative period was measured and compared with that of 25 healthy persons. The results indicate that the blood viscosity in patients is higher than that in the control group (P < 0.001) immediately and one day after operation. Elevation of blood viscosity is not only related to myocardial infarction, but also may be one of the causes for the poor therapeutic effect in patients with carcinomas. It is suggested that proper measures should be taken to control the elevating blood viscosity in patients, especially the elderly ones during the perioperative period. Colloid fluid which is believed to increase blood viscosity should be avoided within the first two days after operation to prevent cardiac complications.