Objective: To discuss the relationship between CO 2 pneumoperitoneum and haemorrheology and the possibility of thrombosis caused by CO 2 . Material and Methods: We compared indices of haemorrheology just prior to operation, 5 min after anaesthesia and 30 min after inflation or operation, and 24 h after operation. The indices included low‐shear and high‐shear whole‐blood relative viscosity; plasma relative viscosity; red cell aggregation index; red cell filtration index; haematocrit; and blood gas analysis. We analysed the relationship between a variation in haemorrheology and CO 2 pneumoperitoneum. Results: After pneumoperitoneum, blood viscosity and red cell aggregation index rises markedly and red cell deformity weakens markedly. Conclusion: Carbon dioxide pneumoperitoneum can cause an increase in blood viscosity, red cell aggregation and a weakening of red cell deformity, and may lead to thrombosis after laparoscopic surgery. image image image image image